参考:http://www.cnblogs.com/duanjie/archive/2011/08/13/2136862.html
说到去重复,感觉逻辑很简单。但动手写起来却并不是那么容易。面试的时候让写sql语句,然后没完整的写出来,然后面试官说你这没结果啊!
去重复首先要把要留下的选出来,然后删除其他重复项。distinct虽然好用,但规则不明显。这里采用:select max(id) from user group by 重复依据。
1.取出要留下的数据,本次依据重复项的最大id为保留项。
SELECT id,name,age from person
WHERE id in(
SELECT MAX(id) from person GROUP BY name,age
);
2.放入临时表
DROP TABLE IF EXISTS temp1;
CREATE TEMPORARY TABLE temp1 SELECT id,name,age from person
WHERE id in(
SELECT MAX(id) from person GROUP BY name,age
);
3.清空原表并从临时表注入
TRUNCATE person;
INSERT INTO person SELECT * from temp1;
DROP TABLE if EXISTS temp1;
SELECT * from person;
当然也可以直接delete:
DELETE from person WHERE id not in (
SELECT mid FROM(
SELECT MAX(id) mid from person GROUP BY name,age
) t
)