一，语法
select 查询列表
from 表名
where 筛选条件
二，筛选条件的分类
1，简单条件运算符
> < = != >= <= <=>安全等于 <>不等于
示例：select * from employee where salary>10000
2，逻辑运算符
&& and
|| or
! not
示例：select * from employee where salary>10000 and salary<20000
3，模糊查询
like
示例：select * from employee where name like '%a%';(name中包含a的显示出来）
select * from employee where name like '__a%';(name中第三个字母是a的显示出来
注：%匹配任意个字符，_匹配任意单个字符，可以用 \ 进行转义
between and
示例：select * from employee where salary between 10000 and 20000
in
示例：select * from employee where salary in {10000,11000,12000}；
is null（用来判断null值，=不能判断null值，<=>可以判断null值）
is not null