數據庫複習之去重數據

1. 查看過濾重複後的數據
思路, group by 分組可以對多個列進行分組, 分組後可以過濾掉重複的數據

SELECT id,un,pw,sex,email FROM USER GROUP BY id

2. 查看重複的數據

SELECT id,un,pw,sex,email,COUNT(1) FROM USER GROUP BY id HAVING COUNT(1)>1 

3. 刪除重複的數據留下一條

DELETE FROM USER
WHERE 1=1
AND id NOT IN (SELECT * FROM ( (SELECT MIN(id) ids FROM USER GROUP BY un HAVING COUNT(un)>1) ) b))