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'
);
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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
)
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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
)
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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
)
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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;
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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