sql笔记

1，分组查询出每组数据的最大值

select Max(id) id
FROM mod_worker_entry_exit GROUP BY roster_id having COUNT(*)>1

2，删除其他数据只保留最大值

DELETE mod_worker_entry_exit
FROM
mod_worker_entry_exit,
( SELECT Max( create_date ) create_date, roster_id FROM mod_worker_entry_exit GROUP BY roster_id HAVING COUNT( * ) > 1 ) t2
WHERE
mod_worker_entry_exit.roster_id = t2.roster_id
AND mod_worker_entry_exit.create_date < t2.create_date;

3

-- 查询每天的统计
SELECT
COUNT(1),
DATE_FORMAT(date,"%y-%m-%d") as date
FROM
mod_worker_entry_exit
GROUP BY DATE_FORMAT(date,"%y-%m-%d");
-- 查询每周的统计
SELECT
COUNT(1),
WEEK(date)as date
FROM
mod_worker_entry_exit
GROUP BY WEEK(date);
-- 查询每月的统计
SELECT
COUNT(1),
MONTH(date)as date
FROM
mod_worker_entry_exit
GROUP BY MONTH(date);
-- 查询每年的统计
SELECT
COUNT(1),
YEAR(date)as date
FROM
mod_worker_entry_exit
GROUP BY YEAR(date);