mysql-常用sql

记录下工作中常用的sql

删除重复数据

delete from student where id not in (select min(id) from student group by name); 
-- 该语句在mysql下会报错，
-- 执行报错：1093 - You can't specify target table 'student' for update in FROM clause
-- 原因是：更新数据时使用了查询，而查询的数据又做了更新的条件，mysql不支持这种方式。外边在包一层即可
 delete from student where id not in (select minid from (select min(id) as minid from student group by name) b);

查找全部重复记录

SELECT * FROM t_info a WHERE ((SELECT COUNT(*) FROM t_info WHERE Title = a.Title) > 1)  -- 重复标题

每组的前N条数据

-- 对上面数据按照日期分组，每组取qty_ordered最大的前5条

SELECT  * FROM 表名 a  WHERE (SELECT count(*) FROM 表名 b WHERE b.日期=a.日期 AND b.id>a.id  )<5;

分组的，某个字段最大

select a.* from table2 as a where age = (select max(age) from table2 where a.table1_id=table1_id);

条件总数

select 
COUNT(1),
COUNT(IF(source = 2 OR source =3 ,TRUE,NULL)) AS zq,
COUNT(IF(source =1,TRUE,NULL))AS jdcount
from table

其他

SELECT SEC_TO_TIME (89    53);

select md5();

select day(now());

DATE_FORMAT( deteline, "%Y-%m-%d %H" );

CONVERT(FROM_BASE64(content),CHAR);

weight + 120 - FLOOR(TIMESTAMPDIFF(DAY,create_time,NOW())/30)*10