m0_73535798的博客

1.增删改2.查询：模糊 like 【% 0个或多个字符 ,_一个字符】3.子查询 select * from sys_menu where parent_id in(select menu_id from sys_menu where parent_id=1)4.级联 select * from sys_user u left join sys_dept sd on u.dept_id = sd.dept_id4.1内联（inner join）两边表数据不能为空时才显示该数据。