sql语句_2

数据表如下

一个user_id对应多个user_name，现在要求是：如果某个用户对应的user_name中存在一个a，打印user_id,a出来；如果不存在，打印user_id,0。打印时候user_id不重复。

SELECT * FROM (
    (SELECT DISTINCT user_id, user_name FROM user1 WHERE user_name = 'a')
    UNION ALL
    (SELECT DISTINCT user_id,'0' AS user_name FROM user1 WHERE user_name <> 'a' AND user_id NOT IN (SELECT DISTINCT user_id FROM user1 WHERE user_name = 'a') )
) AS k

　　输出结果：