SQL常用知识点

1.join多表连接查询

　　inner join：只显示连接条件不为空的数据，如果on后面的连接数据为空 则查询不到。

select a.name, b.name
from
    aaa as a
inner join
    bbb as b
on
    a.id = b.id
where
    a.age = 18 and b.add = 'qwe'

　　left join：以表aaa为主表，会把连接之后aaa表中的数据全部显示出来，即便bbb表中的没有与之对应的id 也会查询出来。

select a.name, b.name
from
    aaa as a
left join
    bbb as b
on
    a.id = b.id
where
    a.age = 18 and b.add = 'qwe'

　　right join：以表bbb为主表，会把连接之后bbb表中的数据全部显示出来，即便aaa表中的没有与之对应的id 也会查询出来。

select a.name, b.name
from
    aaa as a
right join
    bbb as b
on
    a.id = b.id
where
    a.age = 18 and b.add = 'qwe'