子查询：

子查询是将一个查询语句嵌套在另外一个查询语句中，内层查询语句的查询结果，可以作为外来层查询语句提供查询条件。

因此在特定条件下，一个查询语句的条件，需要另外一个查询语句来获取。

前期准备表：

create table employee ( num int(50),

d_idint(50),

namevarchar(50),

ageint(50),

sexvarchar(50),

homeaddvarchar(50)

);insert into employee values(1,1001,‘zhangsan‘,26,‘nan‘,‘beijing‘);insert into employee values(2,1001,‘lisi‘,24,‘nv‘,‘hunan‘);insert into employee values(3,1002,‘wangwu‘,25,‘nan‘,‘jiangsu‘);insert into employee values(4,1004,‘aric‘,15,‘nan‘,‘yingguo‘);select * fromemployee;create table department ( d_id int(50),

d_namevarchar(50),

functionevarchar(50),

addressvarchar(50)

);insert into department values(1001,‘keyanbu‘,‘yanfachanpin‘,‘3lou5hao‘);insert into department values(1002,‘shengchanbu‘,‘shengchanchanp‘,‘5louyiceng‘);insert into department values(1003,‘xiaoshoubu‘,‘cehuaxiaoshou‘,‘1louxiaoshoudating‘);select * from department;

一、带in关键字的子查询

select * from employee where d_id in (select d_id from department );

select * from employee where d_id not in (select d_id from department );

二、带exists关键字的子查询

exists关键字表示存在，使用exists关键字时，内层查询语句不用返回查询的记录。而是返回一个真假值。

如果内层查询语句查询到满足条件的记录，就返回一个真值(true)；否则，返回一个假值(false);

当返回值为true时，外层查询语句将进行查询；而返回false时，外层查询语句不进行查询或者查询不出任何记录。

select * from employee where exists (select d_name from department where d_id = 1003);

select * from employee where exists (select d_name from department where d_id = 1004);

其它：

exists关键字可以与其他查询条件一起使用。条件表达式与exists关键字之间用and或者or来连接。select * from employee where age > 24 and exists (select d_name from department where d_id = 1003);select * from employee where age > 24 and exists (select d_name from department where d_id = 1004);notexists与exists相反。select * from employee where age > 24 and not exists (select d_name from department where d_id = 1003);select * from employee where age > 24 and not exists (select d_name from department where d_id = 1004);

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准备语句：

create table schoarship ( levela int(50),

scoreint(50)

);insert into schoarship(levela,score) values(1,90);insert into schoarship values(2,80);insert into schoarship values(3,70);select * fromschoarship;create table computer_stu ( id int(50),

namevarchar(50),

scoreint(50)

);insert into computer_stu(id,name,score) values (1001,‘lily‘,85);insert into computer_stu(id,name,score) values (1002,‘tom‘,91),

(1003,‘jim‘,87),

(1004,‘aric‘,77),

(1005,‘lucy‘,65),

(1006,‘andy‘,99),

(1007,‘ada‘,85),

(1008,‘jeck‘,70);select * from computer_stu;

select * from schoarship;

select * from computer_stu;

3、带比较运算符的子查询

select id,name,score from computer_stu where score >= (select score from schoarship where levela = 1);/*查询获得一等奖学金的学生有哪些，第一个表为奖学金等级和最低分数*/

select d_id,d_name from department where d_id != (select d_id from employee where age = 24);/*只有生产部和销售部没有年龄等于24岁的员工*/

4、带any关键字的子查询

any关键字表示满足其中任何一个条件。使用any关键字时，只要满足内查询语句返回的结果中的任何一个，就可以通过该条件来执行外层查询语句

select * from computer_stu where score >= any ( select score fromschoarship );/*结果显示，有7个人获得奖学金，只有id为1005的人没有，因为分数为65，不高于奖学金指定最低分数的任何一个*/

5、带all关键字的子查询

all关键字表示满足所有条件。使用all关键字时，只有满足内层查询语句返回的所有结果，才可以执行外层查询语句。

select * from computer_stu where score >= all ( select score fromschoarship );/*结果显示，只有两个人获得奖学金。因为这两个人的分数比所有奖学金要求的分数都高*/