where 之比较运算
> < >= = <= != (select * from students where age = 18)
where 之逻辑运算
and or not (select *from student where age = 18 and ( or ) gender = "女")
not 用例 （查找学生小于18岁 ， 并且性别是女性 select * from students where not age>18 and gender = "女"）
where 之模糊查询
1. like
2.% : 表示任意多个任意字符
3. _ :表示一个任意字符 （例如： select *from students where name = "小%"【%小 ， %小% ， _小 ， __小，__%】）
where 之范围查询 （范围查询分为连续范围查询和非连续范围查询）
in : 非连续 select name from students where age in (18,34);
between ... and ... : select name from students where age between 18 and 34;
where 之空值判断
is null :表示判断值为空 （select * from students where age is null）
is not null :表示判断值非空 (select * from students where age is not null)