面試題: mysql 數據庫去重
mysql去重面試總結
前言：題目大概是這樣的。
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、查找表中多余的重復記錄，重復記錄是根據單個字段（Id）來判斷
網上答案：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、刪除表中多余的重復記錄，重復記錄是根據單個字段（Id）來判斷，只留有rowid最小的記錄
網上答案：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、查找表中多余的重復記錄（多個字段）
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、刪除表中多余的重復記錄（多個字段），只留有rowid最小的記錄
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、查找表中多余的重復記錄（多個字段），不包含rowid最小的記錄
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
)
|
勝負查詢：
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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‘, ‘勝‘);
INSERT INTO `t_game` VALUES (‘2018-03-20‘, ‘勝‘);
INSERT INTO `t_game` VALUES (‘2018-03-20‘, ‘負‘);
INSERT INTO `t_game` VALUES (‘2018-03-21‘, ‘負‘);
INSERT INTO `t_game` VALUES (‘2018-03-21‘, ‘勝‘);
INSERT INTO `t_game` VALUES (‘2018-03-21‘, ‘負‘);
|
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select game_date,(select count(*) from t_game where game_date = t.game_date and game_res = ‘勝‘) as ‘勝‘
,(select count(*) from t_game where game_date = t.game_date and game_res = ‘負‘) as ‘負‘
from t_game as t group by game_date;
|
總結
1、delete不能有別名
2、mysql不支持又查又改，要用臨時表
3、mysql不支持rowid
面試題: mysql 數據庫去重