连表查询
上面两张表经过笛卡尔积获得一个全量拼接的大表；html
笛卡尔积：python
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1
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select
*
from
employee,department;
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内链接(inner join)
双方可以互相匹配的项才会被显示出来；ide
select * from 表1 inner join 表2 on 条件
例如：
select * from employee inner join department
on employee.dep_id = department.id;
表的重命名：post
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1
2
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select t1.name,t2.name
from
employee as t1 inner join department as t2
on t1.dep_id
=
t2.
id
；
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外链接
左外链接
左外链接(left join) 只完整的显示左表中的全部内容,以及右表中与左表匹配的项；url
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select
*
from
表
1
left join 表
2
on 条件
例如：
select
*
from
employee left join department
on employee.dep_id
=
department.
id
；
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右外链接
右外链接(right join) 只完整的显示右表中的全部内容,以及左表中与右表匹配的项；spa
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1
2
3
4
5
|
select
*
from
表
1
right join 表
2
on 条件
例如：
select
*
from
employee right join department
on employee.dep_id
=
department.
id
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全外链接
全外链接 永远显示左表和右表中全部的项，关键字(union)code
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例如：
select
*
from
employee left join department
on employee.dep_id
=
department.
id
union
select
*
from
employee right join department
on employee.dep_id
=
department.
id
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子查询
#1：子查询是将一个查询语句嵌套在另外一个查询语句中。 #2：内层查询语句的查询结果，能够为外层查询语句提供查询条件。 #3：子查询中能够包含：IN、NOT IN、ANY、ALL、EXISTS 和 NOT EXISTS等关键字 #4：还能够包含比较运算符：= 、 !=、> 、<等
# 查"技术"部的全部员工的名字
# select name from employee where dep_id = (select id from department where name = '技术');
# 查"技术"和"销售"部的全部员工的名字
# select name from employee where dep_id in (select id from department where name in ('技术','销售'));
练习：
1.找出年龄大于25岁的员工以及员工所在的部门
select e.name,d.name from employee e inner join department d
on e.dep_id = d.id where e.age>25
2.之内链接的方式查询employee和department表，而且以age字段的升序方式显示
select * from employee e inner join department d
on e.dep_id = d.id order by e.age
1. 查询平均年龄在25岁以上的部门名 select dep_id from employee group by dep_id having avg(age)>25 select name from department where id in (select dep_id from employee group by dep_id having avg(age)>25); 2.查看技术部员工姓名 select name from employee where dep_id = (select id from department where name = '技术'); 3.查看不足1人的部门名(子查询获得的是有人的部门id) 在员工表中不存在的一个部门id,在department表里 在department表里的id字段中找到一个在员工表的dep_id中不存在的项 select name from department where id not in (select dep_id from employee group by dep_id); 把员工表里全部的人所在的dep_id都查出来 子查询
1.查询大于全部人平均年龄的员工名与年龄 select avg(age) from employee; select name,age from employee where age > (select avg(age) from employee); 2.查询大于部门内平均年龄的员工名、年龄 select dep_id,avg(age) from employee group by dep_id; select * from employee inner join (select dep_id,avg(age) as avg_age from employee group by dep_id) as t2 on employee.dep_id = t2.dep_id where employee.age > t2.avg_age; 带比较的子查询