Question

我想知道这些查询，我得到了一份工作，想要列出表user中的所有用户并计算他们的post photo和video 。并且可以选择按ASC或DESC的这些计数限制进行排序我已经尝试过它们但是看到子查询比连接快。我想知道这些查询之间的差异。为什么有时加入比子查询慢。加入最好只有两张桌子？这对我的工作来说最好吗？或者你可以建议另一种更好的解

SUB-QUERY

select
          user.*,
          (select count(*) from post where post.userid = user.id) postCount,
          (select count(*) from photo where photo.userid = user.id) photoCount,
          (select count(*) from video where video.userid = user.id) videoCount
        from user order by postCOunt desc limit $starrow 20

JOIN

SELECT u.id, 
        COUNT(DISTINCT p.id) AS postCount,
        COUNT(DISTINCT ph.id) AS photoCount,
        COUNT(DISTINCT v.id) AS videoCount
    FROM user u
        LEFT JOIN post p
            ON p.userid = u.id
        LEFT JOIN photo ph
            ON ph.userid = u.id
        LEFT JOIN video v
            ON v.userid = u.id
    GROUP BY u.id
    ORDER BY postCount LIMIT $startrow 20

HTML页面中按postCount DESC排序并进行分页的示例。

userid         postCount      photoCount    videCount
1              34             5             4
2              30             12            2
3              21             5             6
4              15             8             4
5              12             15            9
6              8              3             10

.. .. .. ..

Answer 1

您可以使用JOIN

以这种方式进行尝试

SELECT u.id, postCount, photoCount, videoCount
  FROM user u LEFT JOIN 
  (
    SELECT userid, COUNT(*) postCount
      FROM post
     GROUP BY userid
  ) p ON p.userid = u.id LEFT JOIN
  (
    SELECT userid, COUNT(*) photoCount
      FROM photo
     GROUP BY userid
  ) ph ON ph.userid = u.id LEFT JOIN
  (
    SELECT userid, COUNT(*) videoCount
      FROM video
     GROUP BY userid
  ) v ON v.userid = u.id
 ORDER BY postCount 
 LIMIT $startrow, 20

这是从多个表中计数的最佳订购方式？

1 个答案: