这个查询出了什么问题？

Question

我正在选择村庄的总数，从我的表中总人口数来建立统计数据。但是，有一些问题。它返回我的一切（530 pop（共有530个pop），（106个村庄（总共有106个用户））第一行，下一行是NULL

SELECT s1_users.id userid, (

SELECT count( s1_vdata.wref ) 
FROM s1_vdata, s1_users
WHERE s1_vdata.owner = userid
)totalvillages, (

SELECT SUM( s1_vdata.pop ) 
FROM s1_users, s1_vdata
WHERE s1_vdata.owner = userid
)pop
FROM s1_users
WHERE s1_users.dp >=0
ORDER BY s1_users.dp DESC

Answer 1

尝试从内部SELECTS中删除s1_users

Answer 2

您已经在使用INNER JOIN了。您列出以逗号分隔的表格，它是INNER JOIN的快捷方式。

现在，最明显的答案是使用聚合函数（COUNT和SUM）的子查询缺少GROUP BY子句。

SELECT s1_users.id userid, (

SELECT count( s1_vdata.wref ) 
FROM s1_vdata, s1_users
WHERE s1_vdata.owner = userid
GROUP BY s1_vdata.owner
)totalvillages, (

SELECT SUM( s1_vdata.pop ) 
FROM s1_users, s1_vdata
WHERE s1_vdata.owner = userid
GROUP BY s1_vdata.owner
)pop
FROM s1_users
WHERE s1_users.dp >=0
ORDER BY s1_users.dp DESC

但是，使用列列表中的子查询效率非常低。它使子查询在外部查询中为每一行运行一次。

尝试这样做

SELECT 
  s1_users.id AS userid,
  COUNT(s1_vdata.wref) AS totalvillages,
  SUM(s1.vdata.pop) AS pop
FROM
  s1_users, s1_vdata  --I'm cheating here! There's hidden INNER JOIN in this line ;P
WHERE
  s1_users.dp >= 0
  AND s1_users.id = s1_vdata.owner
GROUP BY
  s1_users.id
ORDER BY
  s1_users.dp DESC

Answer 3

SELECT  s1_users.id AS userid,
        (
        SELECT  COUNT(*)
        FROM    s1_vdata
        WHERE   s1_vdata.owner = userid
        ) AS totalvillages,
        (
        SELECT  SUM(pop)
        FROM    s1_vdata
        WHERE   s1_vdata.owner = userid
        ) AS pop
FROM    s1_users
WHERE   dp >= 0
ORDER BY
        dp DESC

请注意，这比此查询效率低：

SELECT  s1_users.id AS user_id, COUNT(s1_vdata.owner), SUM(s1_vdata.pop)
FROM    s1_users
LEFT JOIN
        s1_vdata
ON      s1_vdata.owner = s1_users.id
GROUP BY
        s1_users.id
ORDER BY
        dp DESC

因为聚合需要在前者完成两次。

Answer 4

SELECT userid,totalvillages,pop   from 
(
SELECT s1_users.id as userid, count( s1_vdata.wref ) as totalvillages
FROM s1_vdata, s1_users
WHERE s1_vdata.owner = userid
GROUP BY s1_users.id) tabl1 INNER JOIN
(
SELECT s1_users.id as userid, SUM( s1_vdata.pop )  as pop
FROM s1_users, s1_vdata
WHERE s1_vdata.owner = userid
GROUP BY s1_users.id) tabl2 on tabl1.userid = tabl2.userid