面试题: mysql 数据库去重 已看1 不好使
mysql去重面试总结
前言：题目大概是这样的。
建表：
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CREATE TABLE `test2` ( `id` int(11) NOT NULL AUTO_INCREMENT, `peopleId` int(11) DEFAULT NULL, `name` varchar(255) DEFAULT NULL, PRIMARY KEY (`id`)) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8;-- ------------------------------ Records of test2-- ----------------------------INSERT INTO `test2` VALUES ('1', '1', '倒一');INSERT INTO `test2` VALUES ('2', '1', '倒一');INSERT INTO `test2` VALUES ('3', '3', '等等');INSERT INTO `test2` VALUES ('4', '2', '421');INSERT INTO `test2` VALUES ('5', '2', '421');INSERT INTO `test2` VALUES ('6', '2', '421'); |
1、查找表中多余的重复记录，重复记录是根据单个字段（Id）来判断
网上答案：select * from 表 where Id in (select Id from 表 group byId having count(Id) > 1)
mysql：select * from test2 where id in (select id from test2 group by peopleId having count(peopleId) > 1)
2、删除表中多余的重复记录，重复记录是根据单个字段（Id）来判断，只留有rowid最小的记录
网上答案：DELETE from 表 WHERE (id) IN ( SELECT id FROM 表 GROUP BY id HAVING COUNT(id) > 1) AND ROWID NOT IN (SELECT MIN(ROWID) FROM 表 GROUP BY id HAVING COUNT(*) > 1);
mysql：
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DELETE FROM test2 WHERE peopleId IN( select a.peopleId FROM ( SELECT * FROM test2 GROUP BY peopleId HAVING count(peopleId) > 1 )a )AND id NOT IN ( select b.id FROM ( SELECT * FROM test2 GROUP BY name HAVING count(name) > 1 )b) |
3、查找表中多余的重复记录（多个字段）
select * from 表 a where (a.Id,a.seq) in(select Id,seq from 表 group by Id,seq having count(*) > 1)
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SELECT *FROM test3 aWHERE (a.id, a.seq) IN ( SELECT id, seq FROM test3 GROUP BY id, seq HAVING count(*) > 1 ) |
4、删除表中多余的重复记录（多个字段），只留有rowid最小的记录
delete from 表 a where (a.Id,a.seq) in (select Id,seq from 表 group by Id,seq having count() > 1) and rowid not in (select min(rowid) from 表 group by Id,seq having count()>1)
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DELETE FROM test3 WHERE (id, seq) IN ( SELECT a.id, a.seq FROM ( SELECT id, seq FROM test3 GROUP BY id, seq HAVING count(*) > 1 ) a )AND (id, seq, `name`) NOT IN ( SELECT b.* FROM ( SELECT * FROM test3 GROUP BY id, seq HAVING count(*) > 1 ) b) |
5、查找表中多余的重复记录（多个字段），不包含rowid最小的记录
select * from 表 a where (a.Id,a.seq) in (select Id,seq from 表 group by Id,seq having count() > 1) and rowid not in (select min(rowid) from 表 group by Id,seq having count()>1)
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select * FROM test3 WHERE (id, seq) IN ( SELECT a.id, a.seq FROM ( SELECT id, seq FROM test3 GROUP BY id, seq HAVING count(*) > 1 ) a )AND (id, seq, `name`) NOT IN ( SELECT b.* FROM ( SELECT * FROM test3 GROUP BY id, seq HAVING count(*) > 1 ) b) |
胜负查询：
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CREATE TABLE `t_game` ( `game_date` varchar(255) DEFAULT NULL, `game_res` varchar(255) DEFAULT NULL) ENGINE=InnoDB DEFAULT CHARSET=utf8;-- ------------------------------ Records of t_game-- ----------------------------INSERT INTO `t_game` VALUES ('2018-03-20', '胜');INSERT INTO `t_game` VALUES ('2018-03-20', '胜');INSERT INTO `t_game` VALUES ('2018-03-20', '负');INSERT INTO `t_game` VALUES ('2018-03-21', '负');INSERT INTO `t_game` VALUES ('2018-03-21', '胜');INSERT INTO `t_game` VALUES ('2018-03-21', '负'); |
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select game_date,(select count(*) from t_game where game_date = t.game_date and game_res = '胜') as '胜',(select count(*) from t_game where game_date = t.game_date and game_res = '负') as '负' from t_game as t group by game_date; |
总结
1、delete不能有别名
2、mysql不支持又查又改，要用临时表
3、mysql不支持rowid
