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高考英语必考语法知识点归纳总结免费
英语是高考必考的三门主科之一,所以认真复习英语知识点非常有必要。那么高考英语知识点有哪些呢?以下是小编准备的一些高考英语必考语法知识点归纳,仅供参考。
(一)英语听力:
听力的难点在于第一节只放一遍听力材料,只有一次机会抓取关键信息;同时,听力第二节会存在陷阱,一定要在听完整段材料以后再选答案,不能只听一半就想当然。
(二)英语阅读:
阅读的难点主要在于主旨大意题非常考察学生的总结归纳能力,其混淆选项也是考生失分的重灾区;阅读的细节理解题也非常考察学生的信息定位和查找能力,考生需要注意用时和定位的准确性。
(三)英语七选五:
七选五非常考察考生的快速归纳和信息甄辨能力,回顾检查的难度也较大,而且一道题错的话就至少会有两道题错
(四)英语完形填空:
综合能力要求较高,考察词汇量和语法知识较多,需要从语境中推断关键信息
(五)英语语法填空:
考察的语法知识较多,需要考生自己从上下文找到相关的线索,同时,短文填空需要自己填写单词/单词变体,考察考生的单词拼写
(六)英语写作:
近年新高考改革,作文变成了两道题,题量变大,同时增加了话题的灵活性,增加了续写的新题型,写作难度提升
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高考英语语法必考知识点归纳(完整版)
英语高考本身就不仅仅是背背单词那么简单的,高考英语涉及到很多语法知识,是一次非常全面正式的考试。以下是小编准备的一些高考英语语法必考知识点归纳,仅供参考。
听力应试技巧与策略
听前:略读题目,切入话题,划出重点,预测内容(确定人物身份)
听中:捕捉信息,速记要点,有的放矢,去伪存真(短文独白,首末为主旨句,注意5W,1H)
听后:连贯记忆,前后联系,综合考虑,一锤定音。
阅读理解
1.归纳标题题
特点:短小精悍,一般多为一个短语;涵盖性强,一般能覆盖全文意思;精确性强,表达范围要恰当,不能随意改变语意程度或色彩。常见命题形式有:
What’s the best title for the text?
The best title for this passage is ____.
Which of the following can be the best title for the passage?
2. 概括大意题
包括寻找段落大意(topic)和文章中心思想(main idea),常见命题形式有:
What is the general/main idea of the passage?
Which of the following expresses the main idea?
What is the subject discussed in the text?
细节理解题
考查内容主要涉及时间、地点、人物、事件、原因、结果、数字等议论文中例证细节和定义类细节。这类题目的共同特点是:答案一般都能在文章中找到。当然,答案并不一定是文章中的原句,考生需要根据文章提供的信息自己组织语句回答问题。
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中考英语语法知识归纳总结人教版最新
备考中考英语时,需要对重点语法和一些常考的知识点进行全面的复习。那么中考英语知识点怎么学习呢?以下是小编准备的一些中考英语语法知识归纳总结人教版,仅供参考。
一、听力
听力题主要考查整体理解和获取对话核心信息的能力。应试技巧:1.克服紧张心理,放松自己。心态平稳,才能有效发挥。2.试卷下发后,浏览问题题干,快速预测,以便带着问题有目的地去听。3.集中注意力听录音,果断答题。4.要学会放弃。如果有部分信息确实没听清楚,不要纠缠,马上放弃,以免影响后面答题。5.平时听力训练时要养成简写速记的习惯。
二、完形填空
完形填空题考查词汇知识、阅读能力、逻辑推理判断能力。应试技巧:1.快速通读短文,把握大意。特别注重首尾句的理解,抓住各段的主题句,理顺思路,为后面答题做好铺垫。2.在通读全文、理解大意的基础上,根据固定搭配、习惯用法、上下文暗示先选出一部分答案。然后细读全文,联系上下文内容、短语意思、语法法则、固定搭配等,对选项进行逐一比较和筛选,排除错误选项,最终确定正确答案。3.个别无法确定答案的,先跳过,往往能从下文暗示中找到答案。4.通读全文,验证答案。完成答案后,把所选的答案代入原文,通读全文,对所选答案进行核实调整。重要提示:切忌不通读全文,看一空填一空。
三、阅读理解
阅读理解主要考查学生阅读理解、获取信息和处理信息的能力。应试技巧:1.先看问题,再阅读。带着问题快速阅读全文,把握文章大意,明确作者观点和态度,寻求所需要的信息。2.注重主题句,各段的首尾句。3.概括大意,揣摩文章中心,掌握主旨和作者意图,不要过分推敲语言点。4.遇到生词要根据上下文或语境猜词义,不必停顿时间过长。
四、书面表达
书面表达考查学生综合语言运用能力。应试技巧:写作前:1.审题。审题要仔细,理清写作的体裁和格式,明确信息要点及词数限定等。抓住主旨要义,形成书面表达的思路及框架。2.整编。整合信息要点,分清段落层次,编写出提纲。写作中:1.根据体裁和信息要点明确作文所需要的重点句型和短语,尽量使用自己熟知的句型和短语。遇到不会词句,换用另一种意义相近的句式表达。2.遣词造句,用词地道。3.行文连贯。句与句之间,段与段之间连接自然连贯。写作后:1.检查与修改。重点检查:要点是否全面;内容是否完整;格式是否正确;句型、短语是否准确;语法有否错误;拼写有否错误;字数是否符合要求。2.考生在答卷时,写草稿,或写简略提纲,书写清楚规范,做到零涂改,确保卷面整洁。
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There be句型是英语教学中的一个重点知识点,下面读文网小编为大家带来there be句型的语法教案设计,希望对大家有所帮助。
There be表示 “存在有”,即当我们告诉某人某事存在(或不存在)常用这种结构。There be句型主要用以表达“某处(某时)有某人(某物)。”其基本结构为“There be+某物(某人)+某地(某时)”其中there是引导词,没有词义,其真正的主语在there be 之后。在新课标中对此的考试要求是:掌握There be句式的结构特征和基本用法。在我们的课本中也出现了大量的There be句式如:
There are a few simple safe measures to follow while training.(Book3,P51)
There are also records of gymnastics being performed in China and some other Asian countries.(Book3, P50)
In China, there is only 7% of the land used for growing crops, which feeds more than one fourth of the world population.(Book3, P20)
If the population keeps growing so quickly, there will be only standing room left for us next century.(Book2A, P49)
There were lots of good English language programs broadcast on TV or on the radio in China.(Book1A, P46)
下面这首歌诀可帮你巧记there be句型结构:
There be放句首,主语跟在后。地、时放句末,强调置前头。如:
There is a book on the desk.
有时为了强调地点,也可把介词短语放在句首。如:
On the desk there is a book.
There be句型中的be动词如何确定呢?请先看看下面这首歌诀:
Be动词,有三个,am,is还有are。“There be”真特别,不留am只留俩,那就是is还有are。要用is还是are,须看其后的名词是单数还是复数。若是单数或不可数名词用is,否则就用are。如:
①There is a tree behind the house.
②There is some water(水)in the bottle(瓶子).
③There are some pears in the box.
注意:如果“be”后的主语是由and连接的两个或两个以上的名词,那么be的形式要遵循“远亲不如近邻”的原则。也就是说,“be”的形式是由与它最近的那个名词来确定的。若那个名词是单数或不可数名词要用is,是复数就用are。如:
①There is a book and some pens on the floor.
②There are some pens and a book on the floor.
一、 注意事项:
1 there be 结构中的be 是可以运用各种时态的。
There is going to be a meeting tonight.今晚有个会议。
There was a knock at the door.有人敲门。
There has been a girl waiting for you.有个女孩一直在等你。
There will be rain soon.不久天就要下雨了。
2动词be单复数形式要跟there be之后的真正的主语一致。并且要根据就近一致原则来变换be 的单复数形式。
如:
There is a book on the desk.课桌上有一本书。
How many people are there in the city?这个城市里有多少人口。
There is a pen and two books on the desk.课桌上有一个钢笔和两本书。
There are two books and a pen on the desk. 课桌上有两本书和一个钢笔。
There are some students and a teacher in the classroom. 在教室里有一些学生和一位老师。
There is a teacher and some students in the classroom. 在教室里有一位老师和一些学生。
3 在there be引起的句子结构中,用来修饰主语的不定式主动形式和被动形式均可。
There is no time to lose (= to be lost).时间紧迫。
There is nothing to see (=to be seen).看不见有什么。
There is nothing to do.(=to be done) 无事可做。
二、 结构变形:
在there be 结构中还可把be 改变从而使得there be结构有了一些改变具体总结如下:
1 There used/seem/ happen/appear to be 如:
There might be snow at night.晚上可能有雪。
There appeared to be nobody willing to help.看来没人愿意帮忙。
There used to be a building here.过去这儿有一座楼房。
There happened to be a man walking by.碰巧有个人在此经过。
There doesn't seem to be much hope.好象没有太大的希望。
2 在there be的 be 前还可以加上各种情态词,如:
There must be something wrong.一定有问题。
There ought not to be so many people.不应该有这么多的人。
There might still be hope .可能还有点希望。
3 在there be句型中的be还可以换成其他的动词与there连用,这些词都是表示状态的如:live stand exist remain等或用来描写某事的发生或某人的到达如come, appear, enter, follow, occur等。
There lived a rich man.这以前住着一个富翁。
Then there came a knock at the door.然后有人敲门。
Long, long ago, there lived a king.很久很久以前,有一个国王。
There followed a terrible noise.然后是传来了可怕的声音。
Suddenly there entered a strange man.突然进来了一个奇怪的人。
三、 特殊的表达方式:
1There is no sense in doing 做某事是没有用的,没有意义的
There is no sense in making him angry.跟他生气是没有用的。
There in no sense in going alone.
一个人去是没有好处的。
4 There is no use /good doing 做某事是没有用的,没有必要的
There is no use trying to explain it.解释是没有必要的。
There is no good/use going there.
去那儿是没有好处的。
5 There is no need to do 没有必要做某事
There is no need to worry.
没有必要担心。
There is no need to give him so much money.
根本没有必要给他那么多的钱。
6 There is thought/said/reported to be 人们认为有/据说有/据报道有
There is thought to be an army between in these two countries。
人们认为在这两国之间有一场战争。
There is reported to be a better way to cure cancer.
据报道,找到了一种更好的治疗癌症的方法。
7 There is no doing(口语)不可能…….
There is no telling when he will be back.无法知道他什么时候回来。
There is no knowing what he is doing. 无法知道他在做什么。
四、 there be句式的非限定形式。
There be 的非限定形式有两种,即there to be 和there being。需要掌握以下几个情况:
1作主语
当作主语时,一般是There being结构,当句式中有for时,一般用there to be如:
There being a shop here is a great advantage.
这儿有个商店,真是方便极了。
There being a house with a garden is of great value.
拥有花园的房子是很有价值的。
It is impossible for there to be any more.
不可能再有了。
2 作宾语
作动词宾语时,一般用there to be 结构。常见动词有:expect, mean, intend want, prefer等:
We expect there to be no argument.我们希望不会出现争吵。
People don't want there to be anther war.人们不希望再有战争。
作介词的宾语一般用There being句式,但当是for时一般用there to be句式。
This depended on there being a sudden change. 这需要有一个突然的改变。
The teacher was waiting for there to be complete silence.老师在等着大家都安静下来。
3 作状语
用作状语的there be的形式通常用therebeing结构。
There being no buses, we had to walk home.
由于没有公共汽车,我们不得不走着回家。
There being no nobody in the room, we realized that there was no use crying.
屋里没人,我们意识到哭喊是没有用的。
There being nothing else to do, we went home happily.
由于没事可做,我们快乐地回家了。
注意:如果句中出现for时应用there to be。
It was too late for there to be any buses.
太晚了,没有公共汽车了。
看过there be句型的语法教案设计
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下面是学习英语语法的基础知识哦,来学习下吧!
英语语法基础知识入门教程
音 标
学习提示:
在我们开始学习英语语音音标之前,有一点大家要记住:不要试图用汉语普通话的近似的音去代替英语的音标。例如,有人把英语字母“n’'念成“恩”,用汉语的“e”来代替英语的[e],把字母f、m、x分别念成“爱弗”、“爱木”、“爱克斯”。这是一种很坏的习惯,它使你永远也学不到纯正的英语发音。记住,英语的音标跟汉语的音标缺乏可比性。例如,英语的单元音有十二个之多,而汉语只有六个。即使是两者相似的音,它们在发音的口型、音的长短上都有差别。
音素分类:
英语的音标共有48个,其中元音音标20个,辅音音标28个。相信大家对它们都不陌生了,在正式开始音标学习之前,让我们再来熟悉一下这48个音标。
元音
1、单元音 前元音:[i] [i:] [e] [?]
中元音:[?:] [?] [?]
后元音:[a:] [?] [?:] [u] [u:]
2、双元音 合口双元音:[ei] [ai] [?i] [?u] [au]
集中双元音:[i?] [ε? ] [u?]
辅音
1、爆破音 [p] [b] [t] [d] [k] [g]
2、摩擦音 [f] [v] [θ] [e] [s] [z] [?] [?] [h] [r]
3、破擦音 [ts] [dz] [tr] [dr] [t?] [d?]
4、鼻音 [m] [n] [?]
5、半元音 [j] [w]
6、舌侧音 [l]
音标记忆法:
不要为音标的分类名称操心,你并不需要记住这些名称;也不要为音标的数目太多而发愁,照下面的两个记忆方法去做,你一定能在十分钟内将所有的英语音标记住。
一、元音分组记忆法:将元音音标分成四个大组和四个单个,帮助记忆。
1、四大组
第一组 第二组 第三组 第四组
[i] [i:] [i?] [ei] [ai]
[?] [?:] [?u] [ε?]
[u] [u:] [u?] [au]
[?] [?:] [?i]
记忆窍门:
第一组为“基本型”;第二组为基本型的“延长型”;在基本型后加长音符号;第三组为基本型的“前置型”;第四组为基本型的“后置型”。
2.四单个:一座山[?],一朵花[?],一个[e],一个[a:]
二、辅音成对记忆法:将辅音分成十对和八个单个,帮助记忆。
1.十对
爆破组 摩擦组 破擦组
[p] [b] [f] [v] [ts] [dz]
[t] [d] [θ] [e] [tr] [dr]
[k] [g] [s] [z] [t?] [d?]
[?] [?]
记忆口诀: 十对的辅音清、浊成对,每对的发音部位相同。
2.八单个:[m] [n] [?] [j] [w] [r] [l] [h]
记忆口诀:三鼻音[m] [n] [n], 两半元[w] [j] , 一个[r],一个[l],还有一个[h]。
音节
一、定义:一个元音音素(双元音为一个音素)为一个音节。
1、[ai] [n?u] [st?nd] [ti:t?] 均为单音节。
2、['mi:t?] ['reiz?] 均为双音节。
3、['s?u??list] [repri'zent?tiv]均为多音节。
二、音节的划分:
1、每两个元音之间有一个辅音,该辅音归后。如:['mi:t?] ['reiz?]
2、每两个元音之间有两个辅音,则前后各一个。如:['men??nd] ['f?ktri]
3、每两个元音之间有三个及以上的辅音,依次读出其本身的发音即可。如:
['empti]
简记为:一归后,二分手。
试读:[steid] ['a:ft?] ['men??nd] ['f?ktri] ['s?u??list] [repri'zent?tiv]
三、开、闭、r音节:
1、开音节,又分绝对开音节和相对开音节。
绝对开音节:以发音的元音字母结尾的音节。如:no,we.she。
相对开音节:以元音字母+辅音字母(r除外)+不发音的e结尾的音节。如:these,those,page,life,use。
2、闭音节:以一个或几个辅音字母(r除外)结尾,而中间只有一个元音字母的音节。如:bad,desk,sit,long。
3、,元音字母+r,即 ar er ir or ur
4、元音字母在重读开音节、重读闭音节和r读音节中的读音。
元音字母在重读音节中的读音
字母 开音节 例词 闭音节 例词 r音节 例词
A [ei] name [?] bag [a:] car
E [i:] these [e] pen [?:] her
I [ai] bike [i] his [?:] girl
O [?u] no, note [?] not [?:] for
U [ju:] student [?] bus [?:] nurse
由上表可见:元音字母在重读的开音节中就读字母本身的音。(注意:一切规律都不能包罗万象,如,不符合此规律的词有come, love 等等)
句子成分与句子
第一章 句子成分
构成句子的各个部分叫做句子成分。英语句子主要成分有主语、谓语、表语、宾语、状语、补足语和定语。
1、主语
主语是一个句子的主体,是句子所要说明的人或事情,表示句子描述的是“谁”“什么”。主语一般位于陈述句句首,但在there be结构中主语位于谓语后面。
能作主语的有名词类(含代词、数词);非谓语(含不定式、ing 分词)、从句。
1)名词类
Professor Smith is a famous scientist.史密斯教授是著名的科学家。
We often speak English in class.我们经常在课堂上说英语。
One-third of the students in this class are girls.这个班1/3的学生是女生。
2) 非谓语
To master a foreign language is necessary.掌握一门外语是必要的。
Smoking does harm to the health.吸烟有害健康。
3)从句
When we are going to have an English test has not been decided yet.
我们什么时候进行英语测验还没有决定。
Whether he’ll join us in the discussion is of great importance.
他是否参加我们的讨论是很重要的。
2、谓语
谓语说明主语所做的动作或具有的特征和状态。只有动词在句中才能作谓语,一般放在主语之后。
1)简单谓语
由一个动词或动词短语构成。如:
He practices running every morning.他每天早晨练习跑步。
Yesterday afternoon he reached China.昨天下午他到达桂林。
2)复合谓语
(1)由情态动词或其他助动词加动词原形构成
You may keep the book for two weeks.这本书你可以借两周。
You’d better not take the magazine out of the reading-room.
你最好不要把这本杂志拿出阅览室。
He has caught a bad cold.He has to go to see a doctor.
他患了重感冒,必须去看医生。
3、表语
表语用来说明主语的身份、特征和状态,它一般位于系动词(如be,become,get,look,grow,turn,seem等)之后。
能作表语的有名词类(含代词、数词);形容词;非谓语动词(含分词、不定式、动名词;介词短语、副词及从句。
1)名词
He is an astronaut.他是一位宇航员。
2)形容词
The weather has turned cold.天气变冷了。
3) 非谓语动词
My job is to teach English. 我的工作是教英语。
The speech is exciting.这演讲激动人心。
His hobby is playing football.他的爱好是踢足球。
The teacher was pleased with my spoken English.老师对我的英语口语很满意。
4)介词短语
The machine must be out of order.这机器一定出毛病了。
He is against our plan.他反对我们的计划。
5)副词
Time is up.The class is over.时间到了,下课。
My father isn’t in.He is out.我父亲不在家,他出去了。
6)从句
The truth is that he has never been abroad.实际情况是他从未出过国。
What I want to know is when we’ll have the sports meet.
我想知道的是我们什么时候开运动会。
4、宾语
宾语表示及物动作的对象或承受者,用于回答谓语“谁”和“什么”,一般位于及物动词后面。但英语介词后也要求用宾语。
可充当宾语的内容和主语一样。
1)名词类
They went to see an exhibition yesterday.他们昨天去看了展览。
We should learn from him.我们应该向他学习。
2)非谓语类(不定式、ing 分词)
He pretended not to see me.他假装没有看见我。
She didn’t know what to do next.她不知道下一步做什么。
I enjoy listening to popular music.我喜欢听流行音乐。
3)从句
I think(that)he is fit for his office.我认为他称职。
He asked me whose pronunciation was the best in the class.
他问我班上谁的发音最好。
5、补语
补语用于补充说明主语或宾语。补语通常由形容词、名词或其他相当的结构担任。补语可分为宾语补足语和主语补足语。
1)宾语补足语:
The government appointed her chief delegate to the conference.
政府任命她为出席那个会议的代表团团长。(名词)
He doesn’t believe the story true.
他不相信这故事是真的。 (形容词)
The doctor advised her to stay in bed for a week.
医生建议她卧床休息一周。 (不定式)
They saw her walking into the bookstore.
他们看见她进书店了。 (分词短语)
The children saw the kite up and up.
孩子们看到风筝越飞越高。 (副词)
You should put your books in order.
你应该把书摆整齐。 (介词短语)
2)主语补足语:含宾语补足语的句子改成被动语态时,原宾语和宾语补足语便分别成为主语和主语补足语。
We found him working in the office.(在此主动句子中working是宾补)
我们发现他正在办公室工作。
He was found working in the office.(在此被动句子中working是主补)
他被发现在办公室工作。
6、定语
用来修饰、说明、限定名词或代词的词、短语或从句称为定语。
可充当定语的有形容词;非谓语类(含不定式,ing 分词和ed 分词);名词类(含代词、数词、名词所有格);介词短语、副词和从句。
单个词作定语通常位于所修饰的词之前,但短语、从句作定语时通常位于所修饰的词之后。
1)形容词
Guilin is a beautiful city.桂林是一座美丽的城市。
A bright future shines before us.光明的未来展现在我们眼前。
2) 非谓语类
The building to be built next year will be our school.
明年要建的那幢楼将是我们的学校。
China is a developing country;America is a developed country.
中国是一个发展中国家,美国是一个发达国家。
3)名词类
There are thirty women teachers in our schoo1.我们学校有30名女教师。
More than thirty students in our class have read the book.
我们班三十多个学生读过这本书。
Mary’s parents have gone abroad.玛丽的父母出国了。
4) 介词短语
He is reading an article about how to learn English.
他正在读一篇有关如何学习英语的文章。
Who is the girl in red? 穿红衣服的那个姑娘是谁?
5)副词(多位于被修饰词之后)
A noise outside made him turn around.外面的喧闹声使他转过身来。
The man in the room below is friendly.楼下房间的那人很友好。
6)从句
There is nothing that worries him.没有什么事使他烦恼。
I will remember the day when I first met her.
我将记住第一次遇见她的那一天。
7、状语
修饰动词、形容词、副词或整个句子,说明动作或状态特征的句子成分,叫做状语。
可充当状语的主要有副词、介词短语、非谓语类和从句。
1)副词
Light travels most quickly.光传播得最快。
Factories and buildings are seen here and there.
到处都能见到工厂和建筑物。
2)介词短语
He has lived in the city for ten years.他在那座城市住了10年了。
In spite of the difficulties,we went on with our work.
尽管有困难,但是我们仍继续工作。
3)非谓语类
The box is too heavy for me to lift.这个箱子太重,我抬不起。
He is in the room making a model plane.他在房间里做一架飞机模型。
Not knowing what to do,he decided to ask the teacher for advice.
他不知道怎么办好,就去问老师。
Encouraged by the teacher,I made up my mind to learn English well.
在老师的鼓励下,我决心把英语学好。
4)从句
Once you begin,you must continue.一旦开始,你就得继续下去。
I must work harder in order that I may catch up with the others.
为了赶上其他人,我必须更努力学习。
状语的种类
1)时间状语
How about meeting again at six? 6:00再见面怎样?
2)原因状语
Last night she didn’t go to the dance party because of the rain.
由于下雨,她昨晚没有去参加舞会。
3)条件状语
I shall go there if it doesn’t rain.如果不下雨,我将到那里去。
As(So)long as you work hard,you can make rapid progress in English.
只要你努力,你就可以在英语方面取得很快的进步。
4)地点状语
Mr. Smith lives on the third floor.史密斯先生住在3楼。
Where there is water,there is life.有水的地方,就有生命。
5)方式状语
She put the eggs into the basket with great care.
她小心地把鸡蛋放在篮子里。
He has greatly improved his spoken English by this means.
他用这种方式极大地改善了他的英语口语。
6)伴随状语
She came in with a dictionary in her hand.她进来时手里拿着一本字典。
The teacher came in, followed by a group of students.
老师走了进来,后面跟着一群学生。
7)目的状语
In order to catch up with the others,I must work harder.
为了赶上其他人,我必须更努力学习。
I went there to see a friend of mine.我去那里看我的一个朋友。
8)结果状语
He was so tired that he fell asleep immediately.他累极了,立刻就睡着了。
We arrived there only to find an empty room.
我们到了那儿只发现了一个空房间。
9)让步状语
She works very hard though she is old.
虽然她年纪大了,但她仍然十分努力工作。
No matter when you come,you are warmly welcomed.
不管你什么时候来,都欢迎。
10)程度状语
They were greatly moved to hear the hero’s story.
听了英语的故事,他们深受感动。
I quite agree with you.我完全同意你的意见。
11)比较状语
I am taller than he is.我比他高。
The more I speak English,the better I’ll be.我越多讲英语,就讲得越好。
句子成分练习
指出下列句中画线部分的词性及在句中的作用
1. Tonight we have something special for dinner.
2. a. Thank you for years of your valuable service
b. She keeps her jewels, money and other valuables in the bank
3. He is a very fine musician.
4. a. The farmer enjoys the beauty of the long stretch of his land.
b. My wool sweater stretched when I washed.
5. a. I had a sharp pain in my back.
b. The meeting starts at 3 o’clock sharp.
6. a. Sound travels at 1,000 feet per second.
b. Your idea sounds a good one.
c. Old as Mary is,she still enjoys sound health
7. a. Father booked three seats on a plane.
b. Tom bought the tickets at the booking office outside the cinema.
8. a. On his way home he bought some apples at fruit stand.
b. The injured driver can hardly stand the pain any longer
c .This book stands high in my opinion.
9. To construct a reservoir is an important government project
10. a. Lovely Christmas presents are placed on the floor around a tree.
b. When George left office, the director presented him with a silver teapot.
c. We learn from the past, experience the present, and hope for success in the future.
第二章 句子
第一节 简单句
简单句的五种基本句型
英语句子的主要特征是:是句子就必须含有主、谓。英语简单句有以下五种基本句型。
1.主语+系动词+表语(简称,主系表)
系动词+表语构成名词性合成谓语;可作表语的有名词、名词所有格、代词主格或宾格、名词性物主代词、指示代词、不定代词、形容词、副词、数词、介词短语、现在分词、过去分词、动名词、动词不定式和从句等。
She is a student.(名词)她是个学生。
He seems a clever boy.他似乎是个聪明的男孩。
She looks like her mother.(介词短语)她样子象她母亲。
The film is moving.(现在分词)这影片令人感动。
This shirt is not yours,but hers.(名词性物主代词)
这件衬衫不是你的,是她的。
2.主语+不及物动词(简称,主谓)
The sun rises.太阳升起来了。
He has just come.他刚来。
3.主语+及物动词+宾语(简称,主谓宾)
We love our motherland.我们热爱祖国。
she reads newspapers after supper.她晚饭后读报。
I like swimming.我喜欢游泳。
Li Ping wants to be a doctor.李平想当医生。
4.主语+及物动词+间接宾语+直接宾语(简称,主谓双宾)
直接宾语和间接宾语又叫双宾语。直接宾语指物,间接宾语指人。二者没有逻辑的主谓关系。要求跟双宾语的动词可分为两类:
(1)give,show,send,pass,hand,lend,post,serve,read,take,write,tell,teach.return,wish,allow,offer,promise,throw等。(sth. to sb.)
I lent her a novel.= I lent a novel to her. 我借给她一本小说。
Please show her your photos.= Please show your photos to her.
请把你的照片给她看看。
(2)buy,do, make,find,build, choose,cut,draw, earn,gather,paint,pick, prepare get,order,sing,spare等。(sth. for sb.)
Her mother bought her a red skirt.
= Her mother bought a red skirt for her.
她母亲给她买了一条红裙子。
Please do me a favor.= Please do a favor for me. 请帮个忙。
5.主语+及物动词+宾语+宾补(简称,主谓复合宾)
有些动词带一个宾语意思不完整,还需要一个宾语补足语进一步说明,意思才完整。宾语+宾语补足语构成复合宾语。宾语和宾语补足语之间有逻辑的主谓关系。双宾语之间则没有逻辑的主谓关系。
可作宾语补足语的词类有:名词、形容词、副词、不定式、现在分词、过去分词和介词短语。
常见的要求带宾补的动词有:make,elect,choose,call,name,leave,find, keep, think, set, let,see,show,ask,tell,invite,want,wish,advise,get,force,oblige,allow,permit,like,encourage,order,expect,prefer,hate,persuade等。
Her parents named her Mary.(名词)父母给她取名玛丽。
Do you find life hard here?(形容词)你感觉这儿的生活艰苦吗?
I’m so glad I found you in.(副词)我很高兴你在家。
I strongly advised him not to do so.(不定式)我竭力劝他不要这样做。
Suddenly I felt the atmosphere grow tense.
(不带to 的不定式)我突然感到气氛紧张起来。
I noticed him leaving the house.(-ing形式)我注意到他离开房子。
I was glad to see them so well treated.
(-ed形式)看到他们受到这样好的待遇,我很高兴。
另外,有些动词要求用it作形式宾语,名词或形容词作宾补,如:make,find,feel,think.
I feel it necessary to speak about my opinion.
(形容词)我觉得有必要谈谈我的意见。
思考题
1、什么叫双宾语?
2、什么叫复合宾语?
3、双宾语和复合宾语有什么区别?举例说明。
练习题:说出系列各句分别是哪种句型;如果含有宾补,请划出作宾补的词语。
1、The old man stopped to watch the children playing.
2、She is deeply moved.
3、What I want is this.
4、She is out.
5、Her job is looking after the children.
6、I hope that you can often write me letters.
7、Allow me a few minutes to read the magazine.
8、She offered an old man her seat.
9、She wrote a letter to me.(她写了封信给我。)
10、She wrote a letter for me.(她替我写了封信。)
11、How do you find the dish?
12、Please ask them upstairs.
13、Will you allow me to take this magazine home?
14、I’ve never seen you look so well before.
15、He kept you wait a long time.
16、They invited me to attend the party.
17、They made Smith the president of the University.
18、Doctors recognize Johnson as a leading authority.
19、His teacher advised him to take up the piano.
20、He gave me the facts in brief
第二节 并列句
并列句包括两个或两个以上并列而又相互独立的简单句。其结构为“简单句+连词+简单句”或中间由“;”连接。
1、表示延续、并列关系的连词有and(同,和),so(所以,因此),for, therefore(因为,所以),not only…but(also)(不仅……而且),neither…nor(既不……也不)等。如:
We help them and they help us.
我们帮助他们,他们帮助我们。
He has been working hard, so he has made much progress recently.
他一直努力工作,所以他近来取得了很大进步。
I think,therefore I exist.
我思故我在。
She not only writes her own plays, but she also acts in them.
她不仅自己编剧本,还饰演其中的角色。
He neither speaks English, nor understands it.
他既不会讲英语,也听不懂英语。
2、表示两者之间选择其一常用的有or(或者,否则),otherwise(否则),or else(否则),either…or(不是……就是)。如:
Either he is to blame or I am.
不是他该受责,就是我该受责。
You must go to work or/or else/otherwise you’ll lose your job.
你得去上班了,要不然就要失去这份工作了。
3、表示转折关系的并列连词有but(但是),yet(然而),still(仍然),however(然而),while(而,可是另一方面),whereas(而,反过来)等。
He is well over seventy, but he doesn’t look at all old.
他七十多岁,但看上去一点儿也不老。
She trained hard all year, yet she still failed to reach her best form.
她全年艰苦训练,然而仍未达到自己的最佳状态。
I drink black coffee while he prefers it with cream
我爱喝清咖啡而他喜欢加奶油的。
第三节 复合句
复合句由一个主句和一个或一个以上的从句构成。从句包括名词性从句(主语从句,宾语从句,表语从句)、状语从句和定语从句。 如:
What he said is true. 他所说的是真的。(主语从句)
I didn’t hear what he had said. 我没听见他说的什么。(宾语从句)
The question is who will go there. 问题是谁愿意去那里。(表语从句)
I was about to leave,when the telephone rang.
我正准备离开,电话响了。(状语从句)
This is the book that I bought yesterday. 这是我昨天买的书。(定语从句)
名词性从句、状语从句和定语从句三大从句的具体用法下面我们要分别具体解释。
练习:判断下列句子是并列句还是复合句。
1. The wind blew hard; the snow fell heavily.
2. Use your head,and you’11 find the answer.
3. Do what you’ve been told,otherwise you will be punished.
4. Would you like a cup of coffee or shall we get down to business right away?
5. He is strong,while his brother is weak.
6. He got up early,(and)yet he failed to catch the early bus.
7. He knew what he wanted,however,he didn’t know how to get it.
8. Why he didn’t come wasn’t quite clear.
9. When we’ll have the sports meet hasn’t been decided yet.
10. She is weak, while his son is strong.
11. Though he is old,(yet)he works very hard.
12. I really believe(that)Tom was telling the truth.
13. I am sure that we can get there on time.
14. Do you know the man who is talking to Mr. Jack?
15. Where there is a will, there is a way.
16. Spring comes and trees turn green.
17. He doesn’t smoke,neither/nor does his brother.
18. I spoke so slowly that I could make myself understood.
第四节 名词性从句
一、 定义
首先,请同学们说出下列句各中划线词的词性及语法作用。
Mary is a teacher.
I like English.
结论:它们都是名词,在句中分别做主语、表语和宾语。即在英语中名词的主要作用是做主语、表语和宾语。
然后,我们一起分析下列各句中划线部分的语法作用。
That Mary is a teacher is known to us all.
I know that Mary is a teacher.
The fact is that Mary is a teacher.
结论:它们分别做主语、宾语和表语。均起到了一个名词的作用。再者,划线部分本身就是一个句子,故在复合句中起名词作用的从句统称名词性从句。
二、分类
请重读上述例句并依据它们在全句中的作用,说出它们具体的从句名称。
That Mary is a teacher is known to us all.
I know that Mary is a teacher.
The fact is that Mary is a teacher.
结论:在复合句中起主语作用的从句称为主语从句;
起____作用的从句称为宾语从句;
起表语作用的从句称为____从句;
名词性从句分三类:主语从句;宾语从句和表语从句。
三、连词
引导名词性从句的引导词有三类:
1. that
2. whether/if(主要用于引导主语从句和宾语从句);as if(主要用于引导表语从句)
3. who, whom, whose, what, which, when, where, why, how.
主语从句
that引导的主语从句
That he will come tomorrow is certain.
That she became monitor made us happy.
英文中有一种习惯,把that 引导的主语从句用it来代替它,即it做形式主语,把做真正主语的主语从句放在主句之后。如:
That he will come tomorrow is certain.
=It is certain that he will come tomorrow. 再例:
It is necessary that we learn English well.
It is a pity that we can’t go shopping.
It is said that he has been to England.
whether引导的主语从句
Whether she will be our teacher is not clear.
=It is not clear whether she will be our teacher.
Whether he will succeed or not doesn’t interest me.
=It doesn’t interest me whether he will succeed or not.
wh-词和how 引导的主语从句
(who, whom, whose, what, which, when, where, why均以wh开头,故统称wh-词)
Who will be our English teacher has not been decided.
What you have said sounds reasonable.
注意:1. that在主语从句中不起任何语法作用,但不能省略。
2. if 不能引导主语从句。
宾语从句
它可以作动词谓语的宾语,也可作非谓语动词、某些介词和某些形容词的宾语。
that引导的宾语从句
I really believe(that)Tom was telling the truth.
I am sure that we can get there on time.
由that引起的宾语从句在意思上相当于陈述句,此时that常可省去,但在以下几种情况下,that不可省。
1)that从句被短语,词组等与谓语动词分隔开时,that不可省。
They told us once again that this should never happen.
(此句中that引导的宾语从句被词组once again与主句隔开,因此that不可省。)
Everyone could see,I believe, that Mike was terrified.
(句中I believe为插入语,故that不可省)
2)当连接两个或两个以上的宾语从句时,第二个从句及以后的连接词that不可省。
She said(that)her mother had gone abroad and that she would come back in a week.
whether,if 引导的宾语从句
I don,t know if/whether he has ever been to New York.
I wonder whether he can speak German or not.
以上例句中的whether和if(是否)都是引导宾语从句的,一般情况下whether,和if可互换。但在下列情况下whether和if不可换用。
1)whether之后可紧跟or not或构成whether.. .or not结构,if后一般不能紧跟or not,但可构成if...or not结构.
I wonder whether/if Mr.Smith has arrived or not.
I wonder whether or not Mr.Smith has arrived.(不可用if代替)
2)当宾语从句是否定句时,只可用if而不可用whether。
I don’t care if she doesn’t smile.
3)当该宾语从句为介词宾语时,只可用whether而不可用if.
I’m not interested in whether you are against the proposal or not.
4)当宾语从句中出现了并列连词or构成的并列结构时,只可用whether而不可用if.
He asked me whether I was a student or a teacher.
wh-词和how 引导的宾语从句
I can’t imagine why he did that thing.
Please tell me where you went yesterday.
注意:1. 整个复合句是疑问句时,其中的宾语从句应用陈述句的语序。
2. 当主句的谓语动词是过去时态时,宾语从句的谓语应用过去相应的时态。(表示真理的和客观事实的除外)
表语从句
that引导的表语从句
The reason was that he fell ill.
My idea is that we should spend our holiday in Qingdao.
as if 引导的表语从句
The question is whether it is worth doing.
It looks as if it is going to rain.
wh-词和how 引导的表语从句
Is this where he was born?
The important thing is what on earth has happened to him.
The question is who will go there.
思考题
1.主语从句、宾语从句和表语从句为什么统称为名词性从句?
2.that 引导主语从句和宾语从句有什么异同?
3.whether 和if 在引导名词性从句时的区别是什么?
4.在宾语从句中,哪几种情况that不能省略?在语序和时态方面应注意什么?
练习:
一、找出下列各复合句中的从句并说明是什么从句,且口译全句。
1. It is a wonder that he should know so much about this matter.
2. I’m sure that you are the tallest man in the world.
3. We are afraid that the trip will be too much for an old man.
4. It doesn’t make much difference whether they come or not.
5. Whether or not he is your friend doesn’t too much.
6. Where she has gone is not known yet.
7. You may take whatever you like.
8. He told me where he had been.
9. I know when we will take off for London.
10. How he became a three-good student is known to us a11.
11. Why he didn’t come wasn’t quite clear.
12. It happened that they went out when I called.
13. When we’11 have the sports meet hasn’t been decided yet.
14. Whom the teacher will praise in class is an important question.
15. I’m sorry that I didn’t recognize you just now.
16. We are glad that none of you failed in this exam.
17. He said that he would call on his old teacher.
18. I heard that some of the old teachers in our school had retired.
19. I want to know what has happened to her.
20. Whose answer is correct will be discussed at the meeting.
21. Which composition is the best will be decided tomorrow.
22. I was surprised at what you said.
23. Did he say anything about how the work was to be done?
24. The question is whether it worth doing.
25. The reason is that he fell ill.
二、单项选择
1.Can you tell me_____?
A. who is that gentleman B.that gentleman is who
C. who that gentleman is D.whom is that gentleman
2.Can you tell me _____ the railway station?
A, how I can get to B. how can I get to
C. where I can get to D.where can I get to
3.Ask her _____ come with me.
A. if she will B.if will she
C.whether will she D.will she
4. I have no idea _____ far the airport is from here.
A.what B. how C.it’s D.that
5.They have no idea at all ______ .
A. where he has gone B.where did he go
C. which place has he gone D.where has he gone
6._____ he said is true.
A. What B.That C.Which D.Whether
7._____ you have done might do harm to other people.
A.That B. What C.Which D.This
8.They want to know _____ do to help us.
A. what can they B. what they can
C. how they can D.how can they
9. These photographs will show you ______.
A.what does our village look like B. what our village looks like
C.how does our village look like D.how our village looks like
10. _____ leaves the room last ought to turn off the lights.
A. Anyone B.The person C. Whoever D.Who
11. _____ we need more practice is quite clear.
A.When B.What C. That D./
l2. _____ I accept the gift or refuse it is none of your business
A.If B. Whether C. Even if D.No matter when
13._____ he is doing seems quite difficult.
A.How B.That C.Which D. What
14. ______ that there is another good harvest this year.
A.It says B. It is said C.I was said D.He was said
15. It is suggested that a new building_____.
A. be built at once B. should build next year
C.would be built at once D. could be built next year
16.It is suggested that the experiment _____ under low temperature.
A.makes B.is making C. should be made D.will be made
17._____ was quite helpful.
A. What they advised me to do B.That they advised me to do
C. What did they advise me to do D.All what they advised me to do
18._____ still needs to be discussed.
A. How is the plan to be carried out B. How the plan is to be carried out
C. Why is the plan carried out D. Why the plan is carried out
19.The question is _____ we’11 overcome all the difficulties.
A.what B. how C.that D.which
20.Shanghai is no longer the same city _____ .
A. as it used to be B.what it used to be
C. as it is used to being D.that it used to be
21.Is it possible _____ he misunderstood _____ I said?
A.that, that B. what, what
C.what, that D. that, what
22.He thought _____ he might fail in the exam worried him.
A.which B. that C.when D.so that
23.The fact is ____ he is an advanced worker.
A.what B.which C. that D.why
24.I have no idea _____.
A. what does the word“infinity”mean
B. what the word“infinity”means
C. what the meaning of word the“infinity”
D. what the word“infinity”mean
25.I wonder _____ he asked such a silly question in public.
A.how B.what C.that D. why
26.Are you sure _____?
A. whether she is honest B.that she is honesty
C. she is honest D.is she honest
27.He _____ you are not going abroad.
A.surprised that B. is surprised that
C.surprised at D. is surprised whether
28. What I said was wrong in the last letter for you.I wish I ______ it back.
A. could take B. would take C.will take D.take
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数据库中exists语句的用法的用法你知道吗?下面小编就跟你们详细介绍下数据库中exists语句的用法的用法,希望对你们有用。
SQL中EXISTS的用法
问题:查询学过编号为"01"并且也学过编号为"02"的课程的同学的信息
--方法1
Sql代码
select Student.* from Student , SC where Student.S# = SC.S# and SC.C# = '01' and exists (Select 1 from SC SC_2 where SC_2.S# = SC.S# and SC_2.C# = '02') order by Student.S#
EXISTS用于检查子查询是否至少会返回一行数据,该子查询实际上并不返回任何数据,而是返回值True或False
例表A:TableIn
例表B:TableEx
(二). 比较使用 EXISTS 和 IN 的查询。注意两个查询返回相同的结果。
select * from TableIn where exists(select BID from TableEx where BNAME=TableIn.ANAME)
select * from TableIn where ANAME in(select BNAME from TableEx)
(三). 比较使用 EXISTS 和 = ANY 的查询。注意两个查询返回相同的结果。
select * from TableIn where exists(select BID from TableEx where BNAME=TableIn.ANAME)
select * from TableIn where ANAME=ANY(select BNAME from TableEx)
NOT EXISTS 的作用与 EXISTS 正好相反。如果子查询没有返回行,则满足了 NOT EXISTS 中的 WHERE 子句。
一种通俗的可以理解为:将外查询表的每一行,代入内查询作为检验,如果内查询返回的结果取非空值,则EXISTS子句返回TRUE,这一行行可作为外查询的结果行,否则不能作为结果。
EXISTS与IN的使用效率的问题,通常情况下采用exists要比in效率高,因为IN不走索引,但要看实际情况具体使用:
IN适合于外表大而内表小的情况;EXISTS适合于外表小而内表大的情况。
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数据库in的用法的用法你知道吗?下面小编就跟你们详细介绍下数据库in的用法的用法,希望对你们有用。
MySQL IN 语法
IN 运算符用于 WHERE 表达式中,以列表项的形式支持多个选择,语法如下:
WHERE column IN (value1,value2,...)
WHERE column NOT IN (value1,value2,...)
当 IN 前面加上 NOT 运算符时,表示与 IN 相反的意思,即不在这些列表项内选择。
IN 使用实例
选取 uid 为 2、3、5 的用户数据:
SELECT * FROM user WHERE uid IN (2,3,5)
返回查询结果如下:
uid
username
password
regdate
2 小明 a193686a53e4de85ee3f2ff0576adf01 xiao@163.com 1278063917
3 Jack 0193686a35e4de85ee3f2ff0567adf490 jack@gmail.com 1278061380
5 5idev a193686a53e4de85ee3f2ff0576adf01 5idev@5idev.com 1291107029
IN 子查询
更多情况下,IN 列表项的值是不明确的,而可能是通过一个子查询得到的:
SELECT * FROM article WHERE uid IN(SELECT uid FROM user WHERE status=0)
在这个 SQL 例子里,我们实现了查出所有状态为 0 的用户(可能是被禁止)的所有文章。首先通过一个查询得到所有所有 status=0 的用户:
SELECT uid FROM user WHERE status=0
然后将查询结果作为 IN 的列表项以实现最终的查询结果,注意在子查询中返回的结果必须是一个字段列表项。
IN 运算符补充说明
IN 列表项不仅支持数字,也支持字符甚至时间日期类型等,并且可以将这些不同类型的数据项混合排列而无须跟 column 的类型保持一致:
SELECT * FROM user WHERE uid IN(1,2,'3','c')
一个 IN 只能对一个字段进行范围比对,如果要指定更多字段,可以使用 AND 或 OR 逻辑运算符:
SELECT * FROM user WHERE uid IN(1,2) OR username IN('admin','5idev')
使用 AND 或 OR 逻辑运算符后,IN 还可以和其他如 LIKE、>=、= 等运算符一起使用。
关于 IN 运算符的效率问题
如果 IN 的列表项是确定的,那么可以用多个 OR 来代替:
SELECT * FROM user WHERE uid IN (2,3,5)
https:// 等效为:
SELECT * FROM user WHERE (uid=2 OR aid=3 OR aid=5)
一般认为,如果是对索引字段进行操作,使用 OR 效率高于 IN,但对于列表项不确定的时候(如需要子查询得到结果),就必须使用 IN 运算符。另外,对于子查询表数据小于主查询的时候,也是适用 IN 运算符的。
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数据库中drop的用法的用法你知道吗?下面小编就跟你们详细介绍下数据库中drop的用法的用法,希望对你们有用。
由于数据库表在计算机中占有物理空间,出于效率方面的考滤,在不需要某些数据库表时,可以删除它们.使用drop table语句可以删除数据库表及表中的任何约束,并释放它们所占的物理空间.利用drop table语句删除数据库表的语法结构如下;
drop table [database_name.[schema_name].|schema_name.]table_name][,...n]
各参数的含义如下:
◎database_name:要在其中删除表的数据库的名称.
◎schema_name:表所属架构的名称.
◎table_name:要删除表的名称.
1.删除数据库表 TestTable,实现代码如下所示:
use mydatebase
go
drop table webuser
2.还可以同时删除多个数据库表,表与表之间用逗号隔开,实现代码如下所示:
use mydatebase
go
drop table webuser,UserComment
3.删除临时表,实现代码如下所示:
use mydatebase
go
drop table #webuser,##UserComment
4.在此南昌网络公司技术人员提醒:如果要删除有依赖关联的数据库表,即主键,外键关联表,则要删除两个表之间的关联约束,然后才可以删除表.注意,也可以先删除引用该表的数据库表,实现代码如下所示: 例如:有表产品表关联着产品分类表.我们就要先删除产品表.再来删除产品分类表.
use mydatebase
go
drop table 产品表
删除成功后
use mydatebase
go
drop table 产品分类表
在此南昌网站建设公司工程师提醒:在Oracle中,利用drop table 删除有关联关系的数据库表时,会同时删除与之关联的所有数据库表.
在此顺便介绍在网页编程中Sql语句删除表的表达方式如下所示:
conn.execute("drop TABLE 表名")
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数据库中count的用法的用法你知道吗?下面小编就跟你们详细介绍下数据库中count的用法的用法,希望对你们有用。
count函数是用来统计表中或数组中记录的一个函数,下面我来介绍在MySQL中count函数用法与性能比较吧。count(*) 它返回检索行的数目, 不论其是否包含 NULL值。
SELECT 从一个表中检索,而不检索其它的列,并且没有 WHERE子句时, COUNT(*)被优化到最快的返回速度。
例如:SELECT COUNT(*) FROM student;
COUNT(DISTINCT 字段),返回不同的非NULL值数目;若找不到匹配的项,则COUNT(DISTINCT)返回 0 。
这个优化仅适用于 MyISAM表, 原因是这些表类型会储存一个函数返回记录的精确数量,而且非常容易访问。
对于事务型的存储引擎(InnoDB, BDB), 存储一个精确行数的问题比较多,原因是可能会发生多重事物处理,而每个都可能会对行数产生影响。
例, 创建用于测试的数据表,以进行count数据统计:
CREATE TABLE `user` (
`id` int(5) unsigned NOT NULL AUTO_INCREMENT,
`name` varchar(10) DEFAULT NULL,
`password` varchar(10) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=4 DEFAULT CHARSET=latin1
测试数据为:
1 name1 123456
2 name2 123456
3 name3 123456
4 name4 NULL
请注意以下查询的返回结果:
1,select count(*) from `user`
2,select count(name) from `user`
3,select count(password) from `user`
输出结果:4,4,3
原因分析:
1,count(*)是对行数目进行计数,所以结果为4。
2,count(column_name)是对列中不为空的行进行计数,所以count(name)=4,而count(password)=3。
以上二点,在使用count函数时,要注意下。
使用GROUP BY对每个owner的所有记录分组,没有它,你会得到错误消息:
mysql> SELECT owner, COUNT(*) FROM pet;
ERROR 1140 (42000): Mixing of GROUP columns (MIN(),MAX(),COUNT(),...)
with no GROUP columns is illegal if there is no GROUP BY clause
COUNT( )和GROUP BY以各种方式分类你的数据。下列例子显示出进行动物普查操作的不同方式。
每种动物的数量:
mysql> SELECT species, COUNT(*) FROM pet GROUP BY species;
+---------+----------+
| species | COUNT(*) |
+---------+----------+
| bird | 2 |
| cat | 2 |
| dog | 3 |
| hamster | 1 |
| snake | 1 |
+---------+----------+
每种性别的动物数量:
mysql> SELECT sex, COUNT(*) FROM pet GROUP BY sex;
+------+----------+
| sex | COUNT(*) |
+------+----------+
| NULL | 1 |
| f | 4 |
| m | 4 |
+------+----------+
(在这个输 出中,NULL表示“未知性别”。)
按种类和性别组合的动物数量:
mysql> SELECT species, sex, COUNT(*) FROM pet GROUP BY species, sex;
+---------+------+----------+
| species | sex | COUNT(*) |
+---------+------+----------+
| bird | NULL | 1 |
| bird | f | 1 |
| cat | f | 1 |
| cat | m | 1 |
| dog | f | 1 |
| dog | m | 2 |
| hamster | f | 1 |
| snake | m | 1 |
+---------+------+----------+
若 使用COUNT( ),你不必检索整个表。例如, 前面的查询,当只对狗和猫进行时,应为:
mysql> SELECT species, sex, COUNT(*) FROM pet
-> WHERE species = 'dog' OR species = 'cat'
-> GROUP BY species, sex;
+---------+------+----------+
| species | sex | COUNT(*) |
+---------+------+----------+
| cat | f | 1 |
| cat | m | 1 |
| dog | f | 1 |
| dog | m | 2 |
+---------+------+----------+
或, 如果你仅需要知道已知性别的按性别的动物数目:
mysql> SELECT species, sex, COUNT(*) FROM pet
-> WHERE sex IS NOT NULL
-> GROUP BY species, sex;
+---------+------+----------+
| species | sex | COUNT(*) |
+---------+------+----------+
| bird | f | 1 |
| cat | f | 1 |
| cat | m | 1 |
| dog | f | 1 |
| dog | m | 2 |
| hamster | f | 1 |
| snake | m | 1 |
+---------+------+----------+
顺便提下mysql的DISTINCT的关键字有很多你想不到的用处
1.在count 不重复的记录的时候能用到
比如SELECT COUNT( DISTINCT id ) FROM tablename;
就是计算talbebname表中id不同的记录有多少条
2,在需要返回记录不同的id的具体值的时候可以用
比如SELECT DISTINCT id FROM tablename;
返回talbebname表中不同的id的具体的值
3.上面的情况2对于需要返回mysql表中2列以上的结果时会有歧义
比如SELECT DISTINCT id, type FROM tablename;
实际上返回的是 id与type同时不相同的结果,也就是DISTINCT同时作用了两个字段,必须得id与tyoe都相同的才被排除了,与我们期望的结果不一样
4.这时候可以考虑使用group_concat函数来进行排除,不过这个mysql函数是在mysql4.1以上才支持的
5.其实还有另外一种解决方式,就是使用
SELECT id, type, count(DISTINCT id) FROM tablename
虽然这样的返回结果多了一列无用的count数据(或许你就需要这个我说的无用数据)
返回的结果是 只有id不同的所有结果和上面的4类型可以互补使用,就是看你需要什么样的数据了
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数据库中like的用法的用法你知道吗?下面小编就跟你们详细介绍下数据库中like的用法的用法,希望对你们有用。
语法
match_expression [ NOT ] LIKE pattern [ ESCAPE escape_character ]
参数
match_expression
任何字符串数据类型的有效 SQL Server 表达式。
pattern
match_expression 中的搜索模式,可以包含下列有效 SQL Server 通配符。
通配符 描述 示例
% 包含零个或更多字符的任意字符串。 WHERE title LIKE '%computer%' 将查找处于书名任意位置的包含单词 computer 的所有书名。
_(下划线) 任何单个字符。 WHERE au_fname LIKE '_ean' 将查找以 ean 结尾的所有 4 个字母的名字(Dean、Sean 等)。
[ ] 指定范围 ([a-f]) 或集合 ([abcdef]) 中的任何单个字符。 WHERE au_lname LIKE '[C-P]arsen' 将查找以arsen 结尾且以介于 C 与 P 之间的任何单个字符开始的作者姓氏,例如,Carsen、Larsen、Karsen 等。
[^] 不属于指定范围 ([a-f]) 或集合 ([abcdef]) 的任何单个字符。 WHERE au_lname LIKE 'de[^l]%' 将查找以 de 开始且其后的字母不为 l 的所有作者的姓氏。
Like的运用场合主要在模糊查询的时候,一般以查询字符串居多,这里据一些例子来说他的一般用法:
例1,查询name字段中包含有“明”字的。
select * from table1 where name like ‘%明%’
这里不要使用*来代替,一般在使用0个或者任意个字符构成的字符串的时候最好使用%
不过在首或尾使用可以相互替换,如果在头尾同时使用的话,就必须要使用%
例2,查询name字段中以“李”字开头。
select * from table1 where name like ‘李*’
或者
select * from table1 where name like ‘李%’
例3,查询name字段中含有数字的。
select * from table1 where name like ‘%[0-9]%’
例4,查询name字段中含有小写字母的。
select * from table1 where name like ‘%[a-z]%’
例5,查询name字段中不含有数字的。
select * from table1 where name like ‘%[!0-9]%’
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数据库中sum的用法的用法你知道吗?下面小编就跟你们详细介绍下数据库中sum的用法的用法,希望对你们有用。
根据BOL中解释COUNT用来返回组中的项数。
COUNT(*)返回组中的项数。包括 NULL 值和重复项
COUNT(ALL expression):对组中的每一行都计算 expression 并返回非空值的数量。默认值。
COUNT(DISTINCT expression):对组中的每一行都计算 expression 并返回唯一非空值的数量。
ID Value
----------- --------------------------------------------------
1 A
2 B
2 B
3 B
4 NULL
select COUNT(* ) from Table_1
-----------
5
SELECT COUNT(Value) FROM Table_1
-----------
4
SELECT COUNT(DISTINCT Value) FROM Table_1
-----------
2
SUM返回表达式中所有值的和或仅非重复值的和。SUM 只能用于数字列。空值将被忽略
ID Value
----------- -----------
1 1
2 2
3 3
4 4
5 NULL
6 5
7 5
SELECT SUM(Value) From Table_1
-----------
20
SELECT SUM(DISTINCT Value) From Table_1
-----------
15
以Northwind数据库中的Employees,Orders表为例,列出所有雇员的姓名以及其拥有的Order的个数。分别用Count()和SUM()实现。
SELECT e.EmployeeID,FirstName,LastName,Count(O.EmployeeID) AS OrderCount
FROM dbo.Employees AS E LEFT JOIN dbo.Orders AS O
ON E.EmployeeID = O.EmployeeID
Group By E.EmployeeID,FirstName,LastName
SELECT e.EmployeeID,FirstName,LastName,SUM(CASE WHEN O.EmployeeID IS NULL THEN 0 ELSE 1 END) AS OrderCount
FROM dbo.Employees AS E LEFT JOIN dbo.Orders AS O
ON E.EmployeeID = O.EmployeeID
Group By E.EmployeeID,FirstName,LastName
注意的是这里一定不能用COUNT(*),因为COUNT(*)是计算行数,对于没有定单的用户也是为1。只有在JOIN的情况下,COUNT(*)才等效于Count(O.EmployeeID
附:这两个表的定义。
USE [Northwind]
GO
/****** Object: Table [dbo].[Employees] Script Date: 01/13/2012 22:57:44 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[Employees](
[EmployeeID] [int] IDENTITY(1,1) NOT NULL,
[LastName] [nvarchar](20) NOT NULL,
[FirstName] [nvarchar](10) NOT NULL,
[Title] [nvarchar](30) NULL,
[TitleOfCourtesy] [nvarchar](25) NULL,
[BirthDate] [datetime] NULL,
[HireDate] [datetime] NULL,
[Address] [nvarchar](60) NULL,
[City] [nvarchar](15) NULL,
[Region] [nvarchar](15) NULL,
[PostalCode] [nvarchar](10) NULL,
[Country] [nvarchar](15) NULL,
[HomePhone] [nvarchar](24) NULL,
[Extension] [nvarchar](4) NULL,
[Photo] [image] NULL,
[Notes] [ntext] NULL,
[ReportsTo] [int] NULL,
[PhotoPath] [nvarchar](255) NULL,
CONSTRAINT [PK_Employees] PRIMARY KEY CLUSTERED
(
[EmployeeID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]
GO
ALTER TABLE [dbo].[Employees] WITH NOCHECK ADD CONSTRAINT [FK_Employees_Employees] FOREIGN KEY([ReportsTo])
REFERENCES [dbo].[Employees] ([EmployeeID])
GO
ALTER TABLE [dbo].[Employees] CHECK CONSTRAINT [FK_Employees_Employees]
GO
ALTER TABLE [dbo].[Employees] WITH NOCHECK ADD CONSTRAINT [CK_Birthdate] CHECK (([BirthDate] < getdate()))
GO
ALTER TABLE [dbo].[Employees] CHECK CONSTRAINT [CK_Birthdate]
GO
USE [Northwind]
GO
/****** Object: Table [dbo].[Orders] Script Date: 01/13/2012 23:02:09 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[Orders](
[OrderID] [int] IDENTITY(1,1) NOT NULL,
[CustomerID] [nchar](5) NULL,
[EmployeeID] [int] NULL,
[OrderDate] [datetime] NULL,
[RequiredDate] [datetime] NULL,
[ShippedDate] [datetime] NULL,
[ShipVia] [int] NULL,
[Freight] [money] NULL,
[ShipName] [nvarchar](40) NULL,
[ShipAddress] [nvarchar](60) NULL,
[ShipCity] [nvarchar](15) NULL,
[ShipRegion] [nvarchar](15) NULL,
[ShipPostalCode] [nvarchar](10) NULL,
[ShipCountry] [nvarchar](15) NULL,
CONSTRAINT [PK_Orders] PRIMARY KEY CLUSTERED
(
[OrderID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
ALTER TABLE [dbo].[Orders] WITH NOCHECK ADD CONSTRAINT [FK_Orders_Customers] FOREIGN KEY([CustomerID])
REFERENCES [dbo].[Customers] ([CustomerID])
GO
ALTER TABLE [dbo].[Orders] CHECK CONSTRAINT [FK_Orders_Customers]
GO
ALTER TABLE [dbo].[Orders] WITH NOCHECK ADD CONSTRAINT [FK_Orders_Employees] FOREIGN KEY([EmployeeID])
REFERENCES [dbo].[Employees] ([EmployeeID])
GO
ALTER TABLE [dbo].[Orders] CHECK CONSTRAINT [FK_Orders_Employees]
GO
ALTER TABLE [dbo].[Orders] WITH NOCHECK ADD CONSTRAINT [FK_Orders_Shippers] FOREIGN KEY([ShipVia])
REFERENCES [dbo].[Shippers] ([ShipperID])
GO
ALTER TABLE [dbo].[Orders] CHECK CONSTRAINT [FK_Orders_Shippers]
GO
ALTER TABLE [dbo].[Orders] ADD CONSTRAINT [DF_Orders_Freight] DEFAULT (0) FOR [Freight]
GO
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数据库中select的用法的用法你知道吗?下面小编就跟你们详细介绍下数据库中select的用法的用法,希望对你们有用。
1、select语句可以用回车分隔
?
1
2
3
4
$sql="select * from article where id=1"
和
$sql="select * from article
where id=1",都可以得到正确的结果,但有时分开写或许能更明了一点,特别是当sql语句比较长时
2、批量查询数据
?
1
2
可以用in来实现
$sql="select * from article where id in(1,3,5)"
3、使用concat连接查询的结果
?
1
$sql="select concat(id,"-",con) as res from article where id=1"
返回"1-article content"
4、使用locate
用法:
select locate("hello","hello baby");返回1
不存在返回0
5、使用group by
以前一直没怎么搞明group by 和 order by,其实也满简单的,group by 是把相同的结果编为一组
?
1
exam:$sql="select city ,count(*) from customer group by city";
这句话的意思就是从customer表里列出所有不重复的城市,及其数量(有点类似distinct)
group by 经常与AVG(),MIN(),MAX(),SUM(),COUNT()一起使用
6、使用having
having 允许有条件地聚合数据为组
?
1
2
$sql="select city,count(*),min(birth_day) from customer
group by city having count(*)>10";
这句话是先按city归组,然后找出city地数量大于10的城市
btw:使用group by + having 速度有点慢
同时having子句包含的表达式必须在之前出现过
7、组合子句
where、group by、having、order by(如果这四个都要使用的话,一般按这个顺序排列)
8、使用distinct
distinct是去掉重复值用的
?
1
$sql="select distinct city from customer order by id desc";
这句话的意思就是从customer表中查询所有的不重复的city
9、使用limit
如果要显示某条记录之后的所有记录
?
1
$sql="select * from article limit 100,-1";
10、多表查询
?
1
2
3
4
$sql="select user_name from user u,member m
where u.id=m.id and
m.reg_date>=2006-12-28
order by u.id desc"
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数据库group by的用法的用法你知道吗?下面小编就跟你们详细介绍下数据库group by的用法的用法,希望对你们有用。
1、概述
“Group By”从字面意义上理解就是根据“By”指定的规则对数据进行分组,所谓的分组就是将一个“数据集”划分成若干个“小区域”,然后针对若干个“小区域”进行数据处理。
2、原始表
3、简单Group By
示例1
select 类别, sum(数量) as 数量之和
from A
group by 类别
返回结果如下表,实际上就是分类汇总。
4、Group By 和 Order By
示例2
select 类别, sum(数量) AS 数量之和
from A
group by 类别
order by sum(数量) desc
返回结果如下表
在Access中不可以使用“order by 数量之和 desc”,但在SQL Server中则可以。
5、Group By中Select指定的字段限制
示例3
select 类别, sum(数量) as 数量之和, 摘要
from A
group by 类别
order by 类别 desc
示例3执行后会提示下错误,如下图。这就是需要注意的一点,在select指定的字段要么就要包含在Group By语句的后面,作为分组的依据;要么就要被包含在聚合函数中。
6、Group By All
示例4
select 类别, 摘要, sum(数量) as 数量之和
from A
group by all 类别, 摘要
示例4中则可以指定“摘要”字段,其原因在于“多列分组”中包含了“摘要字段”,其执行结果如下表
“多列分组”实际上就是就是按照多列(类别+摘要)合并后的值进行分组,示例4中可以看到“a, a2001, 13”为“a, a2001, 11”和“a, a2001, 2”两条记录的合并。
SQL Server中虽然支持“group by all”,但Microsoft SQL Server 的未来版本中将删除 GROUP BY ALL,避免在新的开发工作中使用 GROUP BY ALL。Access中是不支持“Group By All”的,但Access中同样支持多列分组,上述SQL Server中的SQL在Access可以写成
select 类别, 摘要, sum(数量) AS 数量之和
from A
group by 类别, 摘要
7、Group By与聚合函数
在示例3中提到group by语句中select指定的字段必须是“分组依据字段”,其他字段若想出现在select中则必须包含在聚合函数中,常见的聚合函数如下表:
函数
作用
支持性
sum(列名) 求和
max(列名) 最大值
min(列名) 最小值
avg(列名) 平均值
first(列名) 第一条记录 仅Access支持
last(列名) 最后一条记录 仅Access支持
count(列名) 统计记录数 注意和count(*)的区别
示例5:求各组平均值
select 类别, avg(数量) AS 平均值 from A group by 类别;
示例6:求各组记录数目
select 类别, count(*) AS 记录数 from A group by 类别;
示例7:求各组记录数目
8、Having与Where的区别
?where 子句的作用是在对查询结果进行分组前,将不符合where条件的行去掉,即在分组之前过滤数据,where条件中不能包含聚组函数,使用where条件过滤出特定的行。
?having 子句的作用是筛选满足条件的组,即在分组之后过滤数据,条件中经常包含聚组函数,使用having 条件过滤出特定的组,也可以使用多个分组标准进行分组。
示例8
select 类别, sum(数量) as 数量之和 from A
group by 类别
having sum(数量) > 18
示例9:Having和Where的联合使用方法
select 类别, SUM(数量)from A
where 数量 gt;8
group by 类别
having SUM(数量) gt; 10
9、Compute 和 Compute By
select * from A where 数量 > 8
执行结果:
示例10:Compute
select *
from A
where 数量>8
compute max(数量),min(数量),avg(数量)
执行结果如下:
compute子句能够观察“查询结果”的数据细节或统计各列数据(如例10中max、min和avg),返回结果由select列表和compute统计结果组成。
示例11:Compute By
select *
from A
where 数量>8
order by 类别
compute max(数量),min(数量),avg(数量) by 类别
执行结果如下:
示例11与示例10相比多了“order by 类别”和“... by 类别”,示例10的执行结果实际是按照分组(a、b、c)进行了显示,每组都是由改组数据列表和改组数统计结果组成,另外:
?compute子句必须与order by子句用一起使用
?compute...by与group by相比,group by 只能得到各组数据的统计结果,而不能看到各组数据
在实际开发中compute与compute by的作用并不是很大,SQL Server支持compute和compute by,而Access并不支持
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情态动词是表示能力,义务,必须,猜测等说话人的语气或情态的动词。下面读文网小编为大家带来情态动词语法讲解及练习题,欢迎大家学习!
1)表过去的可能和许可,(多用于间接引语中)
At that time we thought the story could not be true.
那时我们认为所说的事不可能是真的。
Father said I could swim in the river.
爸爸说我可以在河里游泳。
2)表过去的能力
I could swim when I was only six.
我刚六岁就能游泳。
Could在肯定句中表示过去的能力时,常表抽象的一般的能力。
He could be very naughty when he was a child.
他小时候会是很顽皮的。
3)表“允许”。可表示委婉客气的提出问题或陈述看法
Could I use your bike?
Yes, you can.
他会记得那时吗?
I’m afraid I couldn’t give you an answer today.
恐怕我今天不能回答你。
The teacher said you could go to the store for sweets.
老师说你可以去商店买糖。
3)Could/can+have done 结构表示对过去发生的事情的“怀疑”或“不肯定”。 could 加完成式还用于肯定句时一般表过去可能完成而却未完成的动作。
Can they have won the basketball match?
他们赢了那场篮球赛吗?
What you referred to just now can have made her very sad.
你刚刚所谈到的可能令他很伤心。
You could have completed the task a little earlier.
你本来能早点完成任务的。(但事实上并没有提前完成任务)
I could have passed my examination easily but I made too many stupid mistakes.
我本可以轻易通过考试,但我犯了太多不该犯的错误。
如表具体做某一件事的能力时,则须用 be able to .
He was able to translate the article without a dictionary.
他可以不用词典翻译那篇文章。
Can表示一贯的能力 ,be able to表示客观能力和通过努力可以达到的能力
I can’t swim. But I am sure I will be able to swim through more practicing.
The fire spread through the hotel, but everyone was able to get out
When the boat sank he was able to swim to the bank
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下面读文网小编为大家带来主谓一致语法讲解及练习题,欢迎大家学习
1.并列结构作主语时谓语用复数,例如:
Reading and writing are very important. 读写很重要。
注意:当主语由and连结时,如果它表示一个单一的概念,即指同一人或同一物时,谓语动词用单数,and 此时连接的两个词前只有一个冠词。例如:
The iron and steel industry is very important to our life. 钢铁工业对我们的生活有重要意义。
典型例题
The League secretary and monitor ___ asked to make a speech at the meeting.
A. isB. was C. are D. were
答案B. 注: 先从时态上考虑。这是过去发生的事情应用过去时,先排除A.,C。本题易误选D,因为The League secretary and monitor 好象是两个人,但仔细辨别,monitor 前没有the,在英语中,当一人兼数职时只在第一个职务前加定冠词。后面的职务用and 相连。这样本题主语为一个人,所以应选B。
2. 主谓一致中的靠近原则
1)当there be 句型的主语是一系列事物时,谓语应与最邻近的主语保持一致。例如:
There is a pen, a knife and several books on the desk. 桌上有一支笔、一把小刀和几本书。
There are twenty boy-students and twenty-three girl-students in the class.班上有二十个男孩,二十三个女孩。
2)当either… or… 与neither… nor,连接两个主语时,谓语动词与最邻近的主语保持一致。如果句子是由here, there引导,而主语又不止一个时,谓语通常也和最邻近的主语一致。例如:
Either you or she is to go. 不是你去,就是她去。
Here is a pen, a few envelops and some paper for you. 给你笔、信封和纸。
3.谓语动词与前面的主语一致
当主语有with, together with, like, except, but, no less than, as well as 等词组成的短语时,谓语动词与前面的主语部分一致。例如:
The teacher together with some students is visiting the factory. 教师和一些学生在参观工厂。
He as well as I wants to go boating.他和我想去划船。
4. 谓语需用单数的情况
1)代词each以及由every, some, no, any等构成的复合代词作主语时,或主语中含有each, every时, 谓语需用单数。例如:
Each of us has a tape-recorder. 我们每人都有录音机。
There is something wrong with my watch. 我的表坏了。
2)当主语是一本书或一条格言时,谓语动词常用单数。例如:
The Arabian Night is a book known to lovers of English. 《天方夜谭》是英语爱好者熟悉的一本书。
3)表示金钱,时间,价格或度量衡的复合名词作主语时,通常把这些名词看作一个整体,谓语一般用单数。例如:
Three weeks was allowed for making the necessary preparations. 用三个星期来做准备。
Ten yuan is enough. 十元够了。
5.指代意义决定谓语的单复数
1)代词what, which, who, none, some, any, more, most, all等词的单复数由其指代的词的单复数决定。例如:
All is right.一切顺利。
All are present.人都到齐了。
2)集体名词作主语时,谓语的数要根据主语的意思来决定。如family, audience, crew, crowd, class, company, committee等词后,谓语动词用复数形式时强调这个集体中的各个成员,用单数时强调该集体的整体。例如:
His family isn't very large. 他家成员不多。
His family are music lovers.他家个个都是音乐爱好者。
但集合名词people, police, cattle, poultry等在任何情况下都用复数形式。例如:
Are there any police around? 附近有警察吗?
3)有些名词,如variety, number, population, proportion, majority 等有时看作单数,有时看作复数。例如:
A number of +名词复数+复数动词。
The number of +名词复数+单数动词。
A number of books have lent out.
The majority of the students like English.
6.与后接名词或代词保持一致的情况
1)用half of, most of, none of, heaps of, lots of, plenty of 等引起主语时,谓语动词通常与of后面的名词/代词保持一致。例如:
Most of his money is spent on books. 他大部分的钱化在书上了。
Most of the students are taking an active part in sports. 大部分学生积极参与体育运动。
2)用a portion of, a series of, a pile of, a panel of 等引起主语时,谓语动词通常用单数。例如: A series of accidents has been reported. 媒体报道了一连串的事故。
A pile of lots was set beside the hearth. 炉边有一堆木柴。
3)如 many a 或 more than one 所修饰的短语作主语时,谓语动词多用单数形式。但由more than… of 作主语时,动词应与其后的名词或代词保持一致。例如:
Many a person has read the novel.许多人读过这本书。
More than 60 percent of the students are from the city.百分之六十多的学生来自这个城市。
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数据库in和exists的用法的用法你知道吗?下面小编就跟你们详细介绍下数据库in和exists的用法的用法,希望对你们有用。
SELECT DISTINCT MD001 FROM BOMMD WHERE MD001 NOT IN (SELECT MC001 FROM BOMMC)
NOT EXISTS,exists的用法跟in不一样,一般都需要和子表进行关联,而且关联时,需要用索引,这样就可以加快速度
select DISTINCT MD001 from BOMMD WHERE NOT EXISTS (SELECT MC001 FROM BOMMC where BOMMC.MC001 = BOMMD.MD001)
exists是用来判断是否存在的,当exists(查询)中的查询存在结果时则返回真,否则返回假。not exists则相反。
exists做为where 条件时,是先对where 前的主查询询进行查询,然后用主查询的结果一个一个的代入exists的查询进行判断,如果为真则输出当前这一条主查询的结果,否则不输出。
in和exists
in 是把外表和内表作hash 连接,而exists是对外表作loop循环,每次loop循环再对内表进行查询。一直以来认为exists比in效率高的说法是不准确的。
如果查询的两个表大小相当,那么用in和exists差别不大。
如果两个表中一个较小,一个是大表,则子查询表大的用exists,子查询表小的用in:
例如:表A(小表),表B(大表)1:select * from A where cc in (select cc from B)
效率低,用到了A表上cc列的索引;select * from A where exists(select cc from B where cc=A.cc)
效率高,用到了B表上cc列的索引。
相反的2:select * from B where cc in (select cc from A)
效率高,用到了B表上cc列的索引;select * from B where exists(select cc from A where cc=B.cc)
效率低,用到了A表上cc列的索引。
not in 和not exists如果查询语句使用了not in 那么内外表都进行全表扫描,没有用到索引;而not extsts 的子查询依然能用到表上的索引。所以无论那个表大,用not exists都比not in要快。
in 与 =的区别
select name from student where name in ('zhang','wang','li','zhao');
与
select name from student where name='zhang' or name='li' or name='wang' or name='zhao'
的结果是相同的。
例子如下(即exists返回where后2个比较的where子句中相同值,not exists则返回where子句中不同值):
exists (sql 返回结果集为真)
not exists (sql 不返回结果集为真)
如下:
表A
ID NAME
1 A1
2 A2
3 A3
表B
ID AID NAME
1 1 B1
2 2 B2
3 2 B3
表A和表B是一对多的关系 A.ID --> B.AID
SELECT ID , NAME FROM A WHERE EXISTS (SELECT * FROM B WHERE A.ID = B.AID)
执行结果为
1 A1
2 A2
原因可以按照如下分析
SELECT ID , NAME FROM A WHERE EXISTS (SELECT * FROM B WHERE B.AID = 1)
-->SELECT * FROM B WHERE B.AID = 1有值返回真所以有数据
SELECT ID , NAME FROM A WHERE EXISTS (SELECT * FROM B WHERE B.AID = 2)
-->SELECT * FROM B WHERE B.AID = 2有值返回真所以有数据
SELECT ID , NAME FROM A WHERE EXISTS (SELECT * FROM B WHERE B.AID = 3)
-->SELECT * FROM B WHERE B.AID = 3无值返回真所以没有数据
NOT EXISTS 就是反过来
SELECT ID , NAME FROM A WHERE NOT EXIST (SELECT * FROM B WHERE A.ID = B.AID)
执行结果为
3 A3
sql in与exists区别
IN
确定给定的值是否与子查询或列表中的值相匹配。
EXISTS
指定一个子查询,检测行的存在。
比较使用 EXISTS 和 IN 的查询
这个例子比较了两个语义类似的查询。第一个查询使用 EXISTS 而第二个查询使用 IN。注意两个查询返回相同的信息。
USE pubs
GO
SELECT DISTINCT pub_name
FROM publishers
WHERE EXISTS
(SELECT *
FROM titles
WHERE pub_id = publishers.pub_id
AND type = 'business')
GO
-- Or, using the IN clause:
USE pubs
GO
SELECT distinct pub_name
FROM publishers
WHERE pub_id IN
(SELECT pub_id
FROM titles
WHERE type = 'business')
GO
下面是任一查询的结果集:
pub_name
----------------------------------------
Algodata Infosystems
New Moon Books
(2 row(s) affected)
exits 相当于存在量词:表示集合存在,也就是集合不为空只作用一个集合.例如 exist P 表示P不空时为真; not exist P表示p为空时 为真 in表示一个标量和一元关系的关系。例如:s in P表示当s与P中的某个值相等时 为真; s not in P 表示s与P中的每一个值都不相等时 为真
in和exists
in 是把外表和内表作hash 连接,而exists是对外表作loop循环,每次loop循环再对内表进行查询。
一直以来认为exists比in效率高的说法是不准确的。
如果查询的两个表大小相当,那么用in和exists差别不大。
全文:
in和exists
in 是把外表和内表作hash 连接,而exists是对外表作loop循环,每次loop循环再对内表进行查询。
一直以来认为exists比in效率高的说法是不准确的。
如果查询的两个表大小相当,那么用in和exists差别不大。
如果两个表中一个较小,一个是大表,则子查询表大的用exists,子查询表小的用in:
例如:表A(小表),表B(大表)
1:
select * from A where cc in (select cc from B)
效率低,用到了A表上cc列的索引;
select * from A where exists(select cc from B where cc=A.cc)
效率高,用到了B表上cc列的索引。
相反的
2:
select * from B where cc in (select cc from A)
效率高,用到了B表上cc列的索引;
select * from B where exists(select cc from A where cc=B.cc)
效率低,用到了A表上cc列的索引。
not in 和not exists
如果查询语句使用了not in 那么内外表都进行全表扫描,没有用到索引;
而not extsts 的子查询依然能用到表上的索引。
所以无论那个表大,用not exists都比not in要快。
in 与 =的区别
select name from student where name in ('zhang','wang','li','zhao');
与
select name from student where name='zhang' or name='li' or name='wang' or name='zhao'
的结果是相同的。
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数据库having的用法的用法你知道吗?下面小编就跟你们详细介绍下数据库having的用法的用法,希望对你们有用。
--sql中的group by 用法解析:
-- Group By语句从英文的字面意义上理解就是“根据(by)一定的规则进行分组(Group)”。
--它的作用是通过一定的规则将一个数据集划分成若干个小的区域,然后针对若干个小区域进行数据处理。
--注意:group by 是先排序后分组;
--举例子说明:如果要用到group by 一般用到的就是“每这个字” 例如说明现在有一个这样的表:每个部门有多少人 就要用到分组的技术
select DepartmentID as '部门名称',
COUNT(*) as '个数' from BasicDepartment group by DepartmentID
--这个就是使用了group by +字段进行了分组,其中我们就可以理解为我们按照了部门的名称ID
--DepartmentID将数据集进行了分组;然后再进行各个组的统计数据分别有多少;
--如果不用count(*) 而用类似下面的语法
select DepartmentID,DepartmentName from BasicDepartment group by DepartmentID
--将会出现错误
--消息 8120,级别 16,状态 1,第 1 行
--选择列表中的列 'BasicDepartment.DepartmentName' 无效,因为该列没有包含在聚合函数或 GROUP BY 子句中。
--这就是我们需要注意的一点,如果在返回集字段中,这些字段要么就要包含在Group By语句的后面,
--作为分组的依据;要么就要被包含在聚合函数中。
--出现的错误详解:咱们看看group by 的执行的过程,先执行select 的操作返回一个程序集,
--然后去执行分组的操作,这时候他将根据group by 后面的字段
--进行分组,并且将相同的字段并称一列数据,如果group by 后面没有这个字段的话就要分成好多的数据。
--但是分组就只能将相同的数据分成两列数据,而一列中又只能放入一个字段,所以那些没有进行分组的
--数据系统不知道将数据放入哪里,所以就出现此错误
--目前一种分组情况只有一条记录,一个数据格是无法放入多个数值的,
--所以这里就需要通过一定的处理将这些多值的列转化成单值,然后将其放在对应的
--数据格中,那么完成这个步骤的就是聚合函数。这就是为什么这些函数叫聚合函数(aggregate functions)了
--group by all语法解析:
--如果使用 ALL 关键字,那么查询结果将包括由 GROUP BY 子句产生的所有组,即使某些组没有符合搜索条件的行。
--没有 ALL 关键字,包含 GROUP BY 子句的 SELECT 语句将不显示没有符合条件的行的组。
select DepartmentID,DepartmentName as '部门名称',
COUNT(*) as '个数' from BasicDepartment group by all DepartmentID,DepartmentName
--group by 和having 解释:前提必须了解sql语言中一种特殊的函数:聚合函数,
--例如SUM, COUNT, MAX, AVG等。这些函数和其它函数的根本区别就是它们一般作用在多条记录上。
--WHERE关键字在使用集合函数时不能使用,所以在集合函数中加上了HAVING来起到测试查询结果是否符合条件的作用。
create TABLE Table1
(
ID int identity(1,1) primary key NOT NULL,
classid int,
sex varchar(10),
age int,
)
--添加测试数据
Insert into Table1 values(1,'男',20)
Insert into Table1 values(2,'女',22)
Insert into Table1 values(3,'男',23)
Insert into Table1 values(4,'男',22)
Insert into Table1 values(1,'男',24)
Insert into Table1 values(2,'女',19)
Insert into Table1 values(4,'男',26)
Insert into Table1 values(1,'男',24)
Insert into Table1 values(1,'男',20)
Insert into Table1 values(2,'女',22)
Insert into Table1 values(3,'男',23)
Insert into Table1 values(4,'男',22)
Insert into Table1 values(1,'男',24)
Insert into Table1 values(2,'女',19
--举例子说明:查询table表查询每一个班级中年龄大于20,性别为男的人数
select COUNT(*)as '>20岁人数',classid from Table1 where sex='男' group by classid,age having age>20
--需要注意说明:当同时含有where子句、group by 子句 、having子句及聚集函数时,执行顺序如下:
--执行where子句查找符合条件的数据;
--使用group by 子句对数据进行分组;对group by 子句形成的组运行聚集函数计算每一组的值;最后用having 子句去掉不符合条件的组。
--having 子句中的每一个元素也必须出现在select列表中。有些数据库例外,如oracle.
--having子句和where子句都可以用来设定限制条件以使查询结果满足一定的条件限制。
--having子句限制的是组,而不是行。where子句中不能使用聚集函数,而having子句中可以。
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