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()

我的输出具有相同的列名，因此我可以将结果作为一个整体来处理。但有没有办法限制每个查询2，所以它会抓住2个管理员，2个野心，2个用户？

由于

Answer 1

是的，您可以在每个子查询中添加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，除非您想要有意删除重复项（这会增加额外的处理）。使用limit时，您通常需要order by来指定您获得的行。

Answer 2

试试这个：

SELECT a.id, a.type, a.title, a.message, a.link, a.image, a.colour_scheme 
FROM (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
      ORDER BY RAND() LIMIT 2
    ) AS a
UNION
SELECT b.id, b.type, b.title, b.message, b.link, b.image, b.colour_scheme 
FROM (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
      ORDER BY RAND() LIMIT 2
    ) AS b
UNION 
SELECT c.id, c.type, c.title, c.message, c.link, c.image, c.colour_scheme 
FROM (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() LIMIT 2
    ) AS c

Answer 3

我不熟悉MySQL SQL，但我建议将每个子查询限制为两行。在Ms-sqlserver上，每个选择都会使用“top 2”。为了能够区分子集，为每个子集添加静态值。在第一个选择中，您可以在select语句'type = elder_message'中使用。

有没有办法从每个查询中获取2行？

3 个答案: