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 a WHERE     (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 )

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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