2个带有WHERE子句的LEFT JOINs不起作用

Question

我试图用两个联接都需要WHERE子句进行查询。 我注意到这是行不通的，并且我只能在联接的末尾添加WHERE。 所以我想知道如何才能做到最好。

这是我当前的查询，它在第一个联接中不接受WHERE：

SELECT  
    p.id, p.username, p.date, p.subject, p.year, p.brand, 
    p.model, p.type, p.bodywork, p.text, p.image, p.chassis, COUNT(c.id) AS answers
FROM 
    posts_tbl AS p
LEFT JOIN 
    post_reply_tbl AS c ON c.post_id = p.id WHERE c.type = 1 AND c.type = 3
LEFT JOIN 
    post_reply_tbl AS a ON a.post_id = p.id WHERE a.type = 2
GROUP BY p.id

Answer 1

将其更改为AND而不是WHERE ：

SELECT  p.id, p.username, p.date, p.subject, p.year, p.brand, 
    p.model, p.type, p.bodywork, p.text, p.image, p.chassis, COUNT(c.id) AS answers
FROM posts_tbl AS p
LEFT JOIN post_reply_tbl AS c 
    ON c.post_id = p.id 
    AND c.type = 1 AND c.type = 3
LEFT JOIN post_reply_tbl AS a 
    ON a.post_id = p.id 
    AND a.type = 2
GROUP BY p.id

我认为您需要对此稍作更改，因为c.type不能同时具有两个值：

SELECT  p.id, p.username, p.date, p.subject, p.year, p.brand, 
    p.model, p.type, p.bodywork, p.text, p.image, p.chassis, COUNT(c.id) AS answers
FROM posts_tbl AS p
LEFT JOIN post_reply_tbl AS c 
    ON c.post_id = p.id 
    AND c.type IN (1, 3)  -- changed to use an OR
LEFT JOIN post_reply_tbl AS a 
    ON a.post_id = p.id 
    AND a.type = 2
GROUP BY p.id

如果希望c.type等于1和3 ，则可能需要考虑使用：

SELECT  p.id, p.username, p.date, p.subject, p.year, p.brand, 
    p.model, p.type, p.bodywork, p.text, p.image, p.chassis, c.answers
FROM posts_tbl AS p
LEFT JOIN
(
    select COUNT(c.id) answers, c.post_id
    from post_reply_tbl c
    where c.type in (1, 3)
    group by c.post_id
    having COUNT(distinct type) = 2
) AS c 
    ON c.post_id = p.id 
LEFT JOIN post_reply_tbl AS a 
    ON a.post_id = p.id 
    AND a.type = 2

只是关于查询的后续注释，由于仅在一个列上使用了GROUP BY ，因此MySQL决定返回其他列的值，并且这些值可能是意外的。 您可以确保返回值的唯一方法是通过它们聚合每个列或组。 （请参阅MySQL对GROUP BY的扩展 ）

从MySQL文档中：

MySQL扩展了GROUP BY的使用，以便选择列表可以引用未在GROUP BY子句中命名的非聚合列。 ...您可以使用此功能来避免不必要的列排序和分组，从而获得更好的性能。 但是，这主要在每个未聚合列中未在GROUP BY中命名的所有值对于每个组都相同时才有用。 服务器可以从每个组中自由选择任何值，因此，除非它们相同，否则选择的值是不确定的。 此外，通过添加ORDER BY子句不会影响从每个组中选择值。 选择值之后，将对结果集进行排序，并且ORDER BY不会影响服务器选择的值。

出于这个原因，最好使用子查询来获取count()然后将确保为其余列返回正确的结果。

SELECT  p.id, p.username, p.date, p.subject, p.year, p.brand, 
    p.model, p.type, p.bodywork, p.text, p.image, p.chassis, c.answers
FROM posts_tbl AS p
LEFT JOIN
(
    select COUNT(c.id) answers, c.post_id
    from post_reply_tbl c
    where c.type in (1, 3)
) AS c 
    ON c.post_id = p.id 
LEFT JOIN post_reply_tbl AS a 
    ON a.post_id = p.id 
    AND a.type = 2

Answer 2

您可以在查询末尾添加所有where条件：

SELECT  p.id, p.username, p.date, p.subject, p.year, p.brand, 
    p.model, p.type, p.bodywork, p.text, p.image, p.chassis, COUNT(c.id) AS answers
                FROM posts_tbl AS p
                LEFT JOIN post_reply_tbl AS c ON c.post_id = p.id 
                                LEFT JOIN post_reply_tbl AS a ON a.post_id = p.id 
WHERE a.type = 2
   AND c.type = 1 AND c.type = 3
GROUP BY p.id

或在联接条件中使用AND：

SELECT  p.id, p.username, p.date, p.subject, p.year, p.brand, 
    p.model, p.type, p.bodywork, p.text, p.image, p.chassis, COUNT(c.id) AS answers
                FROM posts_tbl AS p
                LEFT JOIN post_reply_tbl AS c ON c.post_id = p.id AND c.type = 1 AND c.type = 3
                                LEFT JOIN post_reply_tbl AS a ON a.post_id = p.id AND a.type = 2
                                GROUP BY p.id

Answer 3

只需将所有术语放在一个where子句中

SELECT p.id, p.username, p.date, p.subject, p.year, p.brand, 
p.model, p.type, p.bodywork, p.text, p.image, p.chassis, COUNT AS answers
FROM posts_tbl AS p
LEFT JOIN post_reply_tbl AS c ON c.post_id = p.id 
LEFT JOIN post_reply_tbl AS a ON a.post_id = p.id 
WHERE a.type = 2 AND c.type = 1 AND c.type = 3
GROUP BY p.id