通过子查询选择多列

Question

我正在尝试从以下查询中的子查询中选择 2 列，但无法这样做。尝试创建别名表，但仍然无法获取它们。

SELECT
  DISTINCT petid,
  userid,
  (SELECT MAX(comDate) FROM comments WHERE petid=pet.id) AS lastComDate,
  (SELECT userid FROM comments WHERE petid=pet.id ORDER BY id DESC LIMIT 1) AS lastPosterID
FROM 
  pet LEFT JOIN comments ON pet.id = comments.petid
WHERE 
  userid='ABC'      AND 
  deviceID!='ABC'   AND 
  comDate>=DATE_SUB(CURRENT_TIMESTAMP, INTERVAL 2 MONTH);

基本上，我试图从同一行获取lastComDate& lastPosterID- 特定宠物评论中的最新行。请建议我如何以有效的方式获得它们。

上面的查询有效，但由于同一行被提取两次似乎有点矫枉过正。此外，该ORDER BY子句比聚合函数慢得多 - 正如我在分析查询时发现的那样。因此，避免排序的解决方案将不胜感激。

Answer 1

SELECT DISTINCT petid, userid, lastComDate, lastPosterId
FROM 
    pet 
    LEFT JOIN comments ON pet.id = comments.petid 
    LEFT JOIN (
        SELECT MAX(comDate), userid, petid FROM comments GROUP BY userid
    ) a ON a.petid = pet.id
WHERE 
    userid='ABC' 
    AND deviceID!='ABC' 
    AND comDate>=DATE_SUB(CURRENT_TIMESTAMP, INTERVAL 2 MONTH)
;

如果性能在某个地方受到影响，您还可以将子查询提取到临时表中。

Answer 2

鉴于您的表格如下所示：

create table pet (id int, userid int, deviceid int);
create table comments (id int, petid int, comdate date);

这个查询应该可以解决问题：

SELECT 
        p.id, 
        p.userid,
        (SELECT MAX(comDate)
         FROM comments
         WHERE petid = p.id
         AND comDate >= DATE_SUB(
                 CURRENT_TIMESTAMP, INTERVAL 2 MONTH)
               ) AS lastComDate,
        (SELECT userid
         FROM comments
         WHERE petid = p.id
         AND comDate >= DATE_SUB(
              CURRENT_TIMESTAMP, INTERVAL 2 MONTH
         ) ORDER BY id DESC LIMIT 1) AS lastPosterID
    FROM 
        pet p

    WHERE 
        p.userid=1
        AND p.deviceID!=1