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數(shù)據(jù)庫(kù)中表存在重復(fù)數(shù)據(jù)，需要清理重復(fù)數(shù)據(jù)，清理后保留其中一條的情況是比較常見(jiàn)的需求，如何通過(guò)1條SQL準(zhǔn)確的刪除數(shù)據(jù)呢？

1. 創(chuàng)建表及測(cè)試數(shù)據(jù)

1.1 數(shù)據(jù)庫(kù)中創(chuàng)建一張測(cè)試表

CREATE TABLE `test` (
  `id` INT  NOT NULL AUTO_INCREMENT,
  `c1` VARCHAR(20) DEFAULT NULL,
  `c2` VARCHAR(20) DEFAULT NULL,
  `c3` INT  DEFAULT NULL,
  `c4` DATETIME DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=INNODB  DEFAULT CHARSET=utf8;

1.2 插入測(cè)試數(shù)據(jù)

INSERT INTO test(c1,c2,c3,c4) VALUES( 'a','b',10, '2022-05-24 18:00:46'),('a','c',20, '2022-05-24 18:00:46');
INSERT INTO test(c1,c2,c3,c4) VALUES( 'a','c',10, '2022-05-24 18:00:46'),('a','b',20, '2022-05-24 18:00:46');
INSERT INTO test(c1,c2,c3,c4) VALUES( 'b','c',10, '2022-05-24 18:00:46'),('d','b',20, '2022-05-24 18:00:46');
INSERT INTO test(c1,c2,c3,c4) VALUES( 'b','c',20, '2022-05-24 18:00:46'),('d','b',30, '2022-05-24 18:00:46');
INSERT INTO test(c1,c2,c3,c4) VALUES( 'b','c',20, '2022-05-24 18:00:46'),('a','b',40, '2022-05-24 18:00:46');
INSERT INTO test(c1,c2,c3,c4) VALUES( 'd','b',40, '2022-05-24 18:00:46'),('r','f',40, '2022-05-24 18:00:46');

1.3 查看重復(fù)數(shù)據(jù)

例如c1,c2 這2個(gè)字段組合作為唯一條件，則查詢(xún)重復(fù)數(shù)據(jù)的SQL如下

SELECT
  c1,
  c2,
  COUNT(*)
FROM
  test
GROUP BY c1,
  c2
HAVING COUNT(*) > 1;

可見(jiàn)，結(jié)果如下：

2. 如何刪除重復(fù)數(shù)據(jù)

2.1 方案一

很多研發(fā)同學(xué)習(xí)慣的思路如下：

• 先查出重復(fù)的記錄（使用in）
• 再查出在重復(fù)記錄但id不在每組id最大值的記錄
• 直接將select 改為delete進(jìn)行刪除

查詢(xún)SQL如下

SELECT *    FROM  test  
WHERE (c1,c2) IN (
SELECT c1,c2  
FROM test 
GROUP BY c1,c2 
HAVING COUNT(*)>1 )
AND  id NOT  IN (
SELECT MAX(id) 
FROM  test 
GROUP BY c1,c2 
HAVING COUNT(*)>1)
 ORDER BY c1,c2
;

看上去比較符合結(jié)果了，但是改為delete執(zhí)行的時(shí)候結(jié)果如下：

--  delete SQL
DELETE FROM  test  
WHERE (c1,c2) IN (
SELECT c1,c2  
FROM test 
GROUP BY c1,c2 
HAVING COUNT(*)>1 )
AND  id NOT  IN (
SELECT MAX(id) 
FROM  test 
GROUP BY c1,c2 
HAVING COUNT(*)>1)

出現(xiàn)報(bào)錯(cuò)信息：

錯(cuò)誤代碼：1093
You can't specify target table 'test' for update in FROM clause

也就是說(shuō)MySQL里需刪除的目標(biāo)表在in子查詢(xún)中時(shí)，不能直接執(zhí)行刪除操作。

3. 推薦寫(xiě)法

基于以上情況，使用單條SQL刪除的方式如下：

查詢(xún)SQL：

SELECT  a.*  
FROM  test  a ,
(SELECT  c1,c2,MAX(id)id FROM test  GROUP BY c1,c2 HAVING COUNT(*)>1)b
WHERE    a.c1=b.c1 AND a.c2=b.c2
AND a.id <>b.id

刪除SQL

DELETE  a 
FROM  test  a ,
(SELECT  c1,c2,MAX(id)id FROM test  GROUP BY c1,c2 HAVING COUNT(*)>1)b
WHERE    a.c1=b.c1 AND a.c2=b.c2
AND a.id <>b.id

結(jié)果：

<n>查詢(xún)：delete a FROM test a , (select c1,c2,max(id)id from test group by c1,c2 having count(*)>1)b where a.c1=b.c1 and a.c2=b.c2 and a....


共 7 行受到影響

刪除后數(shù)據(jù)如下：

無(wú)重復(fù)數(shù)據(jù)了。