从第二个表中选择最大时间 JOIN

Question

SELECT q.uid, q.title, q.category_id, CONCAT(CONCAT(q.text,'\n'),a.answer) as description
FROM questions q
LEFT JOIN (
     SELECT question_id, crdate, GROUP_CONCAT(text SEPARATOR '|| ') answer
     FROM answers
     GROUP BY question_id
     ORDER BY crdate DESC
) a ON q.uid = a.question_id
WHERE q.deleted = 0 AND q.answers > 0 

我想从第一个表中选择 uid、标题和类别 ID，从第二个表中选择所有文本和 MAX 个答案的 crdate。此查询选择 Min crdate。我应该如何改变它？

UPD 我有表格问题和答案。作为表格的名称，我选择问题和答案。一个问题可能有多个答案。我想将所有答案合并到一个变量中（稍后将一些变量合并到一个变量中以进行全文搜索）并选择最后一个答案的 crdate。

Answer 1

试试这个

SELECT q.uid, q.title, q.category_id, CONCAT(CONCAT(q.text,'\n'),a.answer) as description
FROM questions q
LEFT JOIN (
     SELECT question_id, max(crdate) as crdate, GROUP_CONCAT(text SEPARATOR '|| ') answer
     FROM answers
     GROUP BY question_id
     ORDER BY crdate DESC
) a ON q.uid = a.question_id
WHERE q.deleted = 0 AND q.answers > 0 
GROUP BY q.uid

Answer 2

使用两个连接：

SELECT q.uid, q.title, q.category_id, CONCAT(CONCAT(q.text,'\n'),a.answer) as description
FROM questions q LEFT JOIN 
     (SELECT a.question_id, a.crdate, GROUP_CONCAT(a.text SEPARATOR '|| ') answer
      FROM answers a JOIN
           (SELECT a2.question_id, MAX(a2.crdate) as maxcr
            FROM answers a2
            GROUP BY a2.question_id
           ) a2
           ON a2.question_id = a.question_id and a2.maxcr = a.crdate
      GROUP BY question_id
     ) a
     ON q.uid = a.question_id
WHERE q.deleted = 0 AND q.answers > 0 
GROUP BY q.uid;

最里面的子查询获取问题答案的最大创建日期。中间子查询做你想做的事，将答案连接在一起。

目前尚不清楚外部GROUP BY 在做什么。真的有必要吗？你为什么使用没有聚合函数的GROUP BY？外部SELECT 中的其他列应该使用什么值？您的问题没有提供足够的信息来解决这些问题，但即使在获得最新的crdate 以获取答案后，查询仍然看起来很可疑。