and是左右两边都满足条件? or是只要满足一个条件就可以
案例展示
select *FROM st where (address="杭州" or address="香港") and math>=80;
空值查询
? ? is null?是空 not null 不是空
select *from st where englist is not null;
逻辑查询 > < >=? <=? !=(<>) between? 和not between
select *FROM st where math not between 80 and 90;
select *FROM st where math between 80 and 90;
select *FROM st where math>= 90;
select *FROM st where math<= 90;
select *FROM st where math> 90;
select *FROM st where math< 90;
删除的区别
drop?删除表结构和数据 DDL
delete? 不会删除id自增DML
truncate 会删除id自增? ?会删除约束 DML
select 字段 from 表名 where 查询条件 group by 分组 having 条件=值
order by 字段 排序 desc asc limit start num