SQL语句 菜鸟入门

为了大家更容易理解我举出的SQL语句,本文假定已经建立了一个学生成绩管理数据库,全文均以学生成绩的管理为例来描述。

  1.在查询结果中显示列名:

  a.用as关键字:select name as ‘姓名’ from students order by age

  b.直接表示:select name ‘姓名’ from students order by age

  2.精确查找:

  a.用in限定范围:select * from students where native in (‘湖南’, ‘四川’)

  b.between…and:select * from students where age between 20 and 30

  c.“=”:select * from students where name = ‘李山’

  d.like:select * from students where name like ‘李%’ (注意查询条件中有“%”,则说明是部分匹配,而且还有先后信息在里面,即查找以“李”开头的匹配项。所以若查询有“李”的所有对象,应该命令:’%李% ‘;若是第二个字为李,则应为’_李%’或’_李’或’_李_’。)

  e.[]匹配检查符:select * from courses where cno like ‘[AC]%’ (表示或的关系,与”in(…)”类似,而且”[]”可以表示范围,如:select * from courses where cno like ‘[A-C]%’)

  3.对于时间类型变量的处理

  a.smalldatetime:直接按照字符串处理的方式进行处理,例如:

select * from students where birth > = ‘1980-1-1’ and birth <= ‘1980-12-31’

  4.集函数

  a.count()求和,如:select count(*) from students (求学生总人数)

  b.avg(列)求平均,如:select avg(mark) from grades where cno=’B2’

  c.max(列)和min(列),求最大与最小

  5.分组group

  常用于统计时,如分组查总数:

select gender,count(sno)

from students

group by gender

(查看男女学生各有多少)

  注意:从哪种角度分组就从哪列”group by”

  对于多重分组,只需将分组规则罗列。比如查询各届各专业的男女同学人数 ,那么分组规则有:届别(grade)、专业(mno)和性别(gender),所以有”group by grade, mno, gender”

select grade, mno, gender, count(*)

from students

group by grade, mno, gender

  通常group还和having联用,比如查询1门课以上不及格的学生,则按学号(sno)分类有:

select sno,count(*) from grades

where mark<60

group by sno

having count(*)>1

  6.UNION联合

  合并查询结果,如:

SELECT * FROM students

WHERE name like ‘张%’

UNION [ALL]

SELECT * FROM students

WHERE name like ‘李%’

  7.多表查询

  a.内连接

select g.sno,s.name,c.coursename

from grades g JOIN students s ON g.sno=s.sno

JOIN courses c ON g.cno=c.cno

(注意可以引用别名)

b.外连接

b1.左连接

select courses.cno,max(coursename),count(sno)

from courses LEFT JOIN grades ON courses.cno=grades.cno

group by courses.cno

  左连接特点:显示全部左边表中的所有项目,即使其中有些项中的数据未填写完全。

  左外连接返回那些存在于左表而右表中却没有的行,再加上内连接的行。

  b2.右连接

  与左连接类似

  b3.全连接

select sno,name,major

from students FULL JOIN majors ON students.mno=majors.mno

  两边表中的内容全部显示

  c.自身连接

select c1.cno,c1.coursename,c1.pno,c2.coursename

from courses c1,courses c2 where c1.pno=c2.cno

  采用别名解决问题。

  d.交叉连接

select lastname+firstname from lastname CROSS JOIN firstanme

  相当于做笛卡儿积

  8.嵌套查询

  a.用关键字IN,如查询李山的同乡:

select * from students

where native in (select native from students where name=’ 李山’)

  b.使用关键字EXIST,比如,下面两句是等价的:

select * from students

where sno in (select sno from grades where cno=’B2’)

select * from students where exists

(select * from grades where

grades.sno=students.sno AND cno=’B2’)

  9.关于排序order

  a.对于排序order,有两种方法:asc升序和desc降序

  b.对于排序order,可以按照查询条件中的某项排列,而且这项可用数字表示,如:

select sno,count(*) ,avg(mark) from grades

group by sno

having avg(mark)>85

order by 3

  10.其他

  a.对于有空格的识别名称,应该用”[]”括住。

  b.对于某列中没有数据的特定查询可以用null判断,如select sno,courseno from grades where mark IS NULL

  c.注意区分在嵌套查询中使用的any与all的区别,any相当于逻辑运算“||”而all则相当于逻辑运算“&&”

  d.注意在做否定意义的查询是小心进入陷阱:

  如,没有选修‘B2’课程的学生 :

select students.*

from students, grades

where students.sno=grades.sno

AND grades.cno <> ’B2’

  上面的查询方式是错误的,正确方式见下方:

select * from students

where not exists (select * from grades

where grades.sno=students.sno AND cno=’B2′)

  11.关于有难度多重嵌套查询的解决思想:

  如,选修了全部课程的学生:

select *

from students

where not exists ( select *

from courses

where NOT EXISTS

(select *

from grades

where sno=students.sno

AND cno=courses.cno))

  最外一重:从学生表中选,排除那些有课没选的。用not exist。由于讨论对象是课程,所以第二重查询从course表中找,排除那些选了课的即可

    原文作者:萱哥老了
    原文地址: https://blog.csdn.net/Player26/article/details/2741100
    本文转自网络文章,转载此文章仅为分享知识,如有侵权,请联系博主进行删除。
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