常用SQL

查询：

select ... from ... where ... group by ... having ...;

左连接右连接全连接

left join、 right join、 full join、 inner join 内连接、等值连接

若A有10条数据，B中有15条数据，AB中公共数据有5条

select * from A left join B on ... 结果为10条数据

select * from A right join B on ... 结果为15条数据

select * from A full join B on ... 结果为20条数据

select * from A inner join B on ... 结果为5条数据

exists 与 in

exists用检查子检查的返回值为true或false。

例：select * from A where exists ( select null ) 等价于 select * from A

select * from A where exists (select B.the_id from B where the_id=A.the_id)

select * from A where the_id in (select distinct(the_id) from B)

in只能判断一列，且exists的效率一般比in高，因为in不走索引

isnull(列名，替换字符串)

case when 列名=‘’ then '空' else '非空' end

union 与 union all

A中10条记录，B中15条记录，5条重复记录

select a,b,c from table_A

union

select a,b,c from tbale_B

union默认删除重复记录，所以结果为20条记录

select a,b,c from table_A

union all

select a,b,c from table_B

直接连接两个查询结果，共25条记录