需要重写查询在

Question

我有以下查询二次表中选择多个记录：

SELECT 
      p.id, 
      p.name, 
      tc.comment 
     FROM people AS p 
     LEFT JOIN comments AS tc ON (tc.pID = p.id) 
     WHERE p.projID = $project_id 
     GROUP BY p.id 

当它被写原来曾经有每人一个评论，但后来又改为允许多个来自同一个人的评论。但是，此查询仅挑选与特定人员匹配的第一条评论。我想知道是否有办法改写它，以便它能够选择与每个人相关的所有评论，还是需要将评论分离到它自己的查询中以在每个人的行内运行？这看起来效率很低。

Answer 1

您需要使用GROUP_CONCAT()聚合函数，将返回组合在单个列中的所有注释：

SELECT 
    p.id, 
    p.name, 
    GROUP_CONCAT(tc.comment) AS comments 
FROM 
    people AS p LEFT JOIN comments AS tc 
    ON tc.pID = p.id 
WHERE 
    p.projID = $project_id 
GROUP BY 
    p.id, 
    p.name 

Answer 2

删除GROUP BY，它将返回所有评论。

SELECT 
      p.id, 
      p.name, 
      tc.comment 
     FROM people AS p 
     LEFT JOIN comments AS tc ON (tc.pID = p.id) 
     WHERE p.projID = $project_id