查出谁在重复

查出谁在重复要点

• 按疑似重复列 GROUP BY，COUNT(*) 算每组出现次数
• HAVING 计数 != 1 只留下真正重复的组（组内至少两条）
• 多列重复就把多列都写进 GROUP BY 和 SELECT
• 给计数起别名便于 HAVING 引用，如 HAVING cnt != 1
• 想看重复组里具体有哪些行，再用子查询或 JOIN 捞出
• 先确认'哪些列算重复'再分组，列选错会漏判

查出谁在重复示例

						-- 找出重复的用户名及其出现次数
						SELECT username, COUNT(*) AS cnt
						FROM member
						GROUP BY username
						HAVING cnt != 1;

						-- 按"邮箱加手机"组合判定重复
						SELECT email, phone, COUNT(*) AS cnt
						FROM member
						GROUP BY email, phone
						HAVING cnt != 1;

重复数据处理知识点