mysql - 有没有办法从每个查询中获取 2 行？

Question

我有以下 MYSQL 查询:

SELECT m.id AS id, 'admin' AS type, m.title AS title, m.message AS message, m.link AS link, m.image AS image, 'White' AS colour_scheme
                FROM x_elder_messages m
                UNION
                SELECT a.id AS id, 'evidence' AS type, a.name AS title, a.about AS message, CONCAT('http://www.aaa.com/x/view/ambition/',a.id,'/') AS link, CONCAT('http://www.aaa.com/x/x_images/x_ambitions/current/thumb/',ai.image_id,'.jpg') AS image, 'White' AS colour_scheme
                FROM x_ambitions a
                INNER JOIN x_ambition_images ai
                ON a.id = ai.ambition_id
                UNION 
                SELECT u.id AS id, 'profile_update' AS type, CONCAT(u.x_first_name,' ',u.x_last_name) AS title, CONCAT(CONCAT(u.x_first_name,' ',u.x_last_name),' has recently updated their profile') AS message, CONCAT('http://www.aaa.com/x/view/',u.id,'/') AS link, CONCAT('http://www.aaa.com/x/x_images/x_users/current/thumb/',i.image_id,'.jpg') AS image, 'Dark' AS colour_scheme
                FROM x_user u
                INNER JOIN x_user_images i
                ON u.id = i.user_id
                ORDER BY RAND() 

我让输出具有相同的列名，这样我就可以将结果作为一个整体来处理。但是有什么方法可以限制每个查询的数量为 2，以便它可以获取 2 个管理员、2 个目标和 2 个用户？

谢谢

Answer 1

是的，你可以在每个子查询中放置一个limit:

(SELECT m.id AS id, 'admin' AS type, m.title AS title, m.message AS message, m.link AS link, m.image AS image, 'White' AS colour_scheme
                FROM x_elder_messages m
 LIMIT 2
) UNION
(SELECT a.id AS id, 'evidence' AS type, a.name AS title, a.about AS message, CONCAT('http://www.aaa.com/x/view/ambition/',a.id,'/') AS link, CONCAT('http://www.aaa.com/x/x_images/x_ambitions/current/thumb/',ai.image_id,'.jpg') AS image, 'White' AS colour_scheme
                FROM x_ambitions a
                INNER JOIN x_ambition_images ai
                ON a.id = ai.ambition_id
 LIMIT 2
) UNION 
(SELECT u.id AS id, 'profile_update' AS type, CONCAT(u.x_first_name,' ',u.x_last_name) AS title, CONCAT(CONCAT(u.x_first_name,' ',u.x_last_name),' has recently updated their profile') AS message, CONCAT('http://www.aaa.com/x/view/',u.id,'/') AS link, CONCAT('http://www.aaa.com/x/x_images/x_users/current/thumb/',i.image_id,'.jpg') AS image, 'Dark' AS colour_scheme
                FROM x_user u
                INNER JOIN x_user_images i
                ON u.id = i.user_id
 LIMIT 2
)
ORDER BY RAND() ;

此外，您应该使用union all，除非您想有意删除重复项(这会增加额外的处理)。当使用 limit 时，您通常需要一个 order by 来指定您获得的行数。