1. 查询SQL表中某个字段的重复数据
SELECT user_name,COUNT(*) AS count FROM db_user_info GROUP BY user_name HAVING count > 1; SELECT user_name FROM db_user_info GROUP BY user_name HAVING COUNT(1) > 1; SELECT * FROM db_user_info WHERE user_name IN (SELECT user_name FROM db_user_info GROUP BY user_name HAVING COUNT(1) > 1);
HAVING是分组（GROUP BY）后的筛选条件，分组后的数据组内再筛选；WHERE 则是在分组前筛选。
WHERE 子句中不能使用聚集函数，而 HAVING 子句中可以，所以在集合函数中加上了 HAVING 来起到测试查询结果是否符合条件的作用。即 HAVING 子句的适用场景是可以使用聚合函数
