grammar

1. 执行顺序：

from

where

group by

having

select

distinct

union

order by

1. group by
   

使用分组语句，select 语句后面只能出现分组字段和聚合函数

1. where and having
   

where 用在对 from 后的操作，对行的操作

having 用在对 group by 语句后的操作

1. like
   

%

匹配任意多个字符

_

匹配任意 1 个字符

'xx%'：xx 开头

'%xx%'：包含 xx

'xx_'：xx 后面 1 个字

1. distinct
   

`distinct column_name`

count(distinct, column_name)

1. create drop insert