Question

我正在努力为我的Q＆amp; A网站提供最佳查询。作为一个简化的例子，我有3个表（问题，答案和用户）：

table_users

UserID  FirstName
1       John
2       Jack
3       Ana
4       Mary

table_questions 。

QuestionID  fkUserID Title
1           4        What is...?
2           2        Where is...?
3           1        How to...?
4           3        How much...?

table_answers

AnswerID    fkQuestionID    fkUserID  Answer
1           1               3         It is...
2           2               1         It is located in ZZ...
3           2               4         It is located in YY...
4           3               2         You have to...

我正在创建一个新闻源：

显示按添加日期排序的最后答案
如果有2个对于同一问题的答案，新闻源只会显示最后一个回答（避免重复）。在示例中，新闻源仅显示AnswerID 3（它位于YY ...）并且将跳过AnswerID 2。
新闻源将显示该人的名字创建问题以及回答问题的人，两者都位于同一个用户表中

到目前为止，我在下面创建了这个查询：

我遇到了一些性能/速度问题而且我无法只显示问题的最后一个答案（问题2），在这种情况下，DISTINCT选择无法正常工作。

SELECT 
DISTINCT (A.fkQuestionID) as QuestionID,
Q.Title,
A.AnswerID as AnswerID,
A.Answer,
U1.FirstName as AnswerFirstName,
Q.pkQuestionID,
U2.FirstName as QuestionFirstName

FROM table_answers as A
INNER JOIN table_questions as Q ON Q.QuestionID=A.fkQuestionID
INNER JOIN table_users as U1 ON U1.UserID=A.fkUserID
INNER JOIN table_users as U2 ON U2.UserID=Q.fkUserID
ORDER BY A.AnswerID DESC

我想将预期结果返回为：

QuestionID  AnswerID  AnswerFirstName   QuestionFirstName
3           4         Jack              John
2           3         Mary              Jack
1           1         Ana               Mary

非常欢迎任何帮助。提前谢谢！

Answer 1

您可以在子查询中使用MAX并执行JOIN以获取所需的列：

DEMO

SELECT
    q.QuestionID,
    q.title,
    u1.FirstName AS Asker,
    a.AnswerID,
    a.answer,
    u2.FirstName AS Answerer
FROM table_questions q
INNER JOIN table_users u1
    ON u1.UserID = q.fkUserID
INNER JOIN (
    SELECT 
        fkQuestionID, MAX(AnswerID) AS AnswerID
    FROM table_answers
    GROUP BY fkQuestionID
) t
    ON t.fkQuestionID = q.QuestionID
INNER JOIN table_answers a
    ON a.AnswerID = t.AnswerID
    AND a.fkQuestionID = t.fkQuestionID
INNER JOIN table_users u2
    ON u2.UserID = a.fkUserID

如果您希望包含没有答案的问题，请将INNER JOIN替换为LEFT JOIN。

Answer 2

您可以按fkQuestionID分组以获取max(AnswerID)，然后在join之后使用它。

SELECT 
q.QuestionID,
q.Title,
a1.AnswerID as AnswerID,
a1.Answer,
U1.FirstName as AnswerFirstName,
q.pkQuestionID,
U2.FirstName as QuestionFirstName
FROM
(SELECT MAX(AnswerID) as AnswerID,fkQuestionID FROM table_answers GROUP BY fkQuestionID ) as a2
INNER JOIN table_answers a1  ON a1.fkQuestionID = a2.fkQuestionID AND a1.AnswerID = a2.AnswerID
INNER JOIN table_questions q ON q.QuestionID = a2.fkQuestionID 
INNER JOIN table_users as U1 ON U1.UserID=a1.fkUserID
INNER JOIN table_users as U2 ON U2.UserID=q.fkUserID
ORDER BY a2.AnswerID DESC

Answer 3

Plz试试这个

SELECT  QuestionID ,fkUserID,title,ANS.Answer
FROM    table_questions QST
INNER JOIN (SELECT fkQuestionID,MAX(AnswerID) AnswerID
            FROM    table_answers
            GROUP BY fkQuestionID) ANS ON ANS.fkQuestionID = QST.QuestionID
INNER JOIN table_answers LANS ON LANS.AnswerID = ANS.AnswerID

SQL查询性能和连接表

3 个答案: