distinct 去重複查詢——兩個表join 連接，去掉重複的數據

------distinct 去重複查詢

SELECT
	* from  accounts acc
JOIN (
	SELECT DISTINCT
		accid
	FROM
		ROLES
) r ON r.accid = acc. ID

-----不需要distinct

SELECT
	*
FROM
	(
		SELECT
			MAX (ID) roleid,
			accid
		FROM
			ROLES
		GROUP BY
			accid
	) rr
JOIN (SELECT * FROM accounts) acc ON acc. ID = rr.accid

 

--------解釋一下不用distinct 去重複查詢語句

SELECT
	*
FROM
	(
		SELECT
			MAX (不重複的字段就行) AS roleid,
			要去重複字段名
		FROM
			數據表 Where  條件
		GROUP BY
			  要去重複字段名
	) AS A
JOIN 數據庫表 ON 條件