mysql删除重复记录并且只保留一条的实现方法

CREATE TABLE `tab` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(20) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=13 DEFAULT CHARSET=utf8;
 
-- ----------------------------
-- Records of tab
-- ----------------------------
INSERT INTO `tab` VALUES ('1', 'A');
INSERT INTO `tab` VALUES ('2', 'A');
INSERT INTO `tab` VALUES ('3', 'A');
INSERT INTO `tab` VALUES ('4', 'B');
INSERT INTO `tab` VALUES ('5', 'B');
INSERT INTO `tab` VALUES ('6', 'C');
INSERT INTO `tab` VALUES ('7', 'B');
INSERT INTO `tab` VALUES ('8', 'B');
INSERT INTO `tab` VALUES ('9', 'B');
INSERT INTO `tab` VALUES ('10', 'E');
INSERT INTO `tab` VALUES ('11', 'E');
INSERT INTO `tab` VALUES ('12', 'E');

SELECT `name`,COUNT(1) FROM TAB GROUP BY `name` HAVING COUNT(1) >1

SELECT `name`,id  FROM TAB GROUP BY `name`  HAVING COUNT(1) >1

DELETE from tab where
-- 删除所有的重复时间 Begin --
`name` in (
    SELECT * from (SELECT `name`FROM TAB GROUP BY `name`  HAVING COUNT(1) >1) tmp2
)
-- 删除所有的重复时间 END --
-- 但一些特定ID的记录不进行删除  Begin --
AND
id NOT in(
    select id from (
        SELECT `name`,id  FROM TAB GROUP BY `name` HAVING COUNT(1) >1
    ) tmp1
)
-- 但一些特定ID的记录不进行删除  END --

-- 方法一
SELECT * FROM t_user WHERE user_name IN (
	SELECT user_name FROM t_user GROUP BY user_name HAVING COUNT(1)>1
) 
AND id NOT IN (
	SELECT MIN(id) FROM t_user GROUP BY user_name HAVING COUNT(1)>1
)

-- 方法二
SELECT * FROM t_user WHERE id NOT IN (
	SELECT MIN(id) FROM t_user GROUP BY user_name
)

-- 方法三
SELECT * FROM t_user AS t1 WHERE t1.id <> (
	SELECT MAX(t2.id) FROM t_user AS t2 WHERE t1.user_name=t2.user_name
)

-- 方法一（笨方法但容易理解）
DELETE FROM t_user WHERE user_name IN (
	SELECT t1.user_name FROM (
		-- 查询出所有重复的user_name
		SELECT user_name FROM t_user GROUP BY user_name HAVING COUNT(1)>1
	) t1
) 
AND id NOT IN (
	SELECT t2.min_id FROM (
		-- 查询出所有重复的记录并各自只取其中一条（MIN(id)或MAX(id)都可以）
		SELECT MIN(id) AS min_id FROM t_user GROUP BY user_name HAVING COUNT(1)>1
	) t2
)
-- 方法二（推荐方法也容易理解）
DELETE FROM t_user WHERE id NOT IN (
	SELECT t.min_id FROM (
		-- 过滤出重复多余的数据，比如，如果所有记录中存在1条记录是user_name=zhangsan的，那么就取出它；
    	-- 如果所有记录中存在多条记录是user_name=lisi的，那么只取其中1条，其他的不查询出来
		SELECT MIN(id) AS min_id FROM t_user GROUP BY user_name
  ) t
)
-- 方法三（推荐方法但不太容易理解）
DELETE FROM t_user WHERE id IN (
	SELECT t.id FROM (
		-- 1. 关于所有存在相同user_name的记录，只查询出（保留）重复记录中的1条，假设这样查询出来的集合为A集合。
		-- 2. 在所有记录中，只要id不在A集合中的，都把它们查询出来
		SELECT t1.id FROM t_user AS t1 WHERE t1.id <> (SELECT MAX(t2.id) FROM t_user AS t2 WHERE t1.user_name=t2.user_name)
	) t
)
-- 或
DELETE FROM t_user t1
WHERE t1.id <> (
	SELECT t2.max_id FROM (
		SELECT MAX(t3.id) AS max_id FROM t_user t3 WHERE t1.user_name=t3.user_name
	) t2
)