数据库高级语句

1、查询一个表中重复的字段

SELECT 字段 AS 重复的,COUNT(字段) AS 相同的 FROM table GROUP BY 字段 HAVING COUNT(字段)>1

2、删除表中重复的字段

id_是记录的主键   string是判断重复的字段，例如学生的身份证号：f_sfzh

DELETE FROM w_jcxs0101 WHERE id_ NOT IN(SELECT a.id_ AS id_ FROM 
(SELECT MIN(id_) AS id_ FROM w_jcxs0101 GROUP BY STRING) a )