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
-- 刪除所有的重復時間 Begin --
`name` in (
    SELECT * from (SELECT `name`FROM TAB GROUP BY `name`  HAVING COUNT(1) >1) tmp2
)
-- 刪除所有的重復時間 END --
-- 但一些特定ID的記錄不進行刪除  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的記錄不進行刪除  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 (
		-- 查詢出所有重復的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 (
		-- 查詢出所有重復的記錄并各自只取其中一條（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（推薦方法也容易理解）
DELETE FROM t_user WHERE id NOT IN (
	SELECT t.min_id FROM (
		-- 過濾出重復多余的數(shù)據(jù)，比如，如果所有記錄中存在1條記錄是user_name=zhangsan的，那么就取出它；
    	-- 如果所有記錄中存在多條記錄是user_name=lisi的，那么只取其中1條，其他的不查詢出來
		SELECT MIN(id) AS min_id FROM t_user GROUP BY user_name
  ) t
)
-- 方法三（推薦方法但不太容易理解）
DELETE FROM t_user WHERE id IN (
	SELECT t.id FROM (
		-- 1. 關于所有存在相同user_name的記錄，只查詢出（保留）重復記錄中的1條，假設這樣查詢出來的集合為A集合。
		-- 2. 在所有記錄中，只要id不在A集合中的，都把它們查詢出來
		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
)