------distinct 去重复查询
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
--------解释一下不用distinct 去重复查询语句
SELECT
*
FROM
(
SELECT
MAX (不重复的字段就行) AS roleid,
要去重复字段名
FROM
数据表 Where 条件
GROUP BY
要去重复字段名
) AS A
JOIN 数据库表 ON 条件