表user：
1.(1)全部查询：select * from user
(2)模糊查询：select * from user where time like '%2019-10-10%'
（3）条件查询：select * from user where id=1
(4)非空查询：select * from user where name is not null and name!=''
（5）去重查询：select distinct name from user
2.添加：insert into user(name,sex,time) values('11',1,now())
3.修改：update user set name='12' where id=11
4.删除：delete from user where id=11
5.别名：select id as userid from user（或select id userid from user）
6.（1）分组查性别数量：select sex,count(sex) countsex from user group by sex
（2）分组条件查询性别数量：select sex,count(sex) countsex from user group by sex HAVING count(sex)<6
7.查询今天的数据：select * from user where to_days(time) = to_days(now());
8.查询30天之内的数据：select * from user where DATE_SUB(CURDATE(), INTERVAL 30 DAY) <= date(time)
9.查询本月的数据：select * from user where DATE_FORMAT( time, '%Y%m' ) = DATE_FORMAT( CURDATE( ) , '%Y%m' )
10.id求和：select sum(id) from user
11.id求平均值：select AVG(id) from user
12.截取字符串：
（1）按关键字截取字符串（SUBSTRING_INDEX（字段，关键字，关键字出现的次数））：select SUBSTRING_INDEX(time,'-',2) from user
（2）从左开始截取字符串：left(被截取字段，截取长度)：select LEFT(time,4) from user
（3）从右开始截取字符串：right(被截取字段，截取长度)：select right(time,4) from user
（4）从第几位开始截取字符串：substring(被截取字段，从第几位开始截取)substring(被截取字段，从第几位开始截取，截取长度)（如果位数是负数 如-5 则是从后倒数位数）：select substring(time,3) from user ； select substring(time,3,4) from user ； select substring(time,-3,4) from user
13.排序查询（desc倒序，asc升序(只有一个排序条件可以省略)）：select * from user order by id desc