mysql 常用SQl

获取行号

SELECT
(@rowNum:=@rowNum+1) as rowNo ,T.*
FROM oic_service_project T,(Select (@rowNum :=0) ) b
where T.APPLY_ID = 1

删除三个月之前的数据

delete from table1 where date1<curdate() - interval 3 month;

修改数据库表自增的值

delete from table1 where date1<curdate() - interval 3 month;

mysql默认的事务处理级别是'REPEATABLE-READ',也就是可重复读1.查看当前会话隔离级别select @@tx_isolation;2.查看系统当前隔离级别select @@global.tx_isolation;3.设置当前会话隔离级别set session transaction isolatin level repeatable read;4.设置系统当前隔离级别set global transaction isolation level repeatable read;
计算索引的选择性SELECT count(DISTINCT(CREATE_DATE))/count(*) AS Selectivity FROM tpl_action_t;