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我对 SQL 还是很陌生，我很难理解为什么这些查询中的一个可以正常工作而一个不能正常工作。这源于我尝试优化复杂而缓慢的查询。@kalengi建议了对我来说似乎是一个绝妙的解决方案，但它似乎不适用于我的网站。以下是查询。

这是 WordPress 生成的标准 SQL（按预期工作）：

SELECT SQL_CALC_FOUND_ROWS wp_posts.ID
FROM wp_posts 
INNER JOIN wp_postmeta ON (wp_posts.ID = wp_postmeta.post_id)
INNER JOIN wp_postmeta AS mt1 ON (wp_posts.ID = mt1.post_id)
WHERE 1=1 
AND wp_posts.post_type = 'product'
AND (wp_posts.post_status = 'publish')
AND (
   (wp_postmeta.meta_key = '_visibility' AND CAST(wp_postmeta.meta_value AS CHAR) IN ('visible','catalog'))
   AND  (mt1.meta_key = '_stock_status' AND CAST(mt1.meta_value AS CHAR) = 'instock')
)
GROUP BY wp_posts.ID
ORDER BY wp_posts.menu_order,wp_posts.post_title asc
LIMIT 0, 10

这是@kalengi 的过滤器处理它以将多个 INNER JOIN 合并为一个之后的 SQL（返回 0 个结果）：

SELECT SQL_CALC_FOUND_ROWS wp_posts.ID
FROM wp_posts 
INNER JOIN wp_postmeta AS pmta ON (wp_posts.ID = pmta.post_id) 
WHERE 1=1 
AND wp_posts.post_type = 'product'
AND ( wp_posts.post_status = 'publish' )
AND (
  ( pmta.meta_key = '_visibility' AND CAST(pmta.meta_value AS CHAR) IN ( 'visible','catalog' ) )
  AND ( pmta.meta_key = '_stock_status' AND CAST(pmta.meta_value AS CHAR) = 'instock' )
)
GROUP BY wp_posts.ID
ORDER BY wp_posts.menu_order,wp_posts.post_title asc
LIMIT 0, 10

谁能解释为什么第二个对我不起作用？

4

2 回答 2

1

看起来您的WHERE子句中有一个条件同时在pmta.meta_keyandpmta.meta_value列中查找两个值：

....
( pmta.meta_key = '_visibility' AND CAST(pmta.meta_value AS CHAR) IN ('visible','catalog' ) )
AND ( pmta.meta_key = '_stock_status' AND CAST(pmta.meta_value AS CHAR) = 'instock' )
....

一列不能在同一行中有两个不同的值，因此此测试将返回 FALSE，因此不会返回任何行。

如果您重写原始查询以将连接条件分组到ON子句中，您可以看到为什么您的第二个查询不起作用：

SELECT 
    SQL_CALC_FOUND_ROWS wp_posts.ID
FROM 
    wp_posts 
    INNER JOIN wp_postmeta 
        ON wp_posts.ID = wp_postmeta.post_id
        AND wp_postmeta.meta_key = '_visibility' 
        AND CAST(wp_postmeta.meta_value AS CHAR) IN ('visible','catalog')
    INNER JOIN wp_postmeta AS mt1 
        ON wp_posts.ID = mt1.post_id
        AND mt1.meta_key = '_stock_status' 
        AND CAST(mt1.meta_value AS CHAR) = 'instock'
WHERE 
    1=1 
    AND wp_posts.post_type = 'product'
    AND wp_posts.post_status = 'publish'
GROUP BY 
    wp_posts.ID
ORDER BY 
    wp_posts.menu_order,
    wp_posts.post_title asc
LIMIT 0, 10

如果您只想加入表一次，请尝试以下操作：

SELECT 
    SQL_CALC_FOUND_ROWS wp_posts.ID
FROM 
    wp_posts 
    INNER JOIN wp_postmeta 
        ON wp_posts.ID = wp_postmeta.post_id
        AND (
            wp_postmeta.meta_key = '_visibility' 
            AND CAST(wp_postmeta.meta_value AS CHAR) IN ('visible','catalog')
        ) OR (
            wp_postmeta.meta_key = '_stock_status' 
            AND CAST(wp_postmeta.meta_value AS CHAR) = 'instock'
        )
WHERE 
    1=1 
    AND wp_posts.post_type = 'product'
    AND wp_posts.post_status = 'publish'
GROUP BY 
    wp_posts.ID
ORDER BY 
    wp_posts.menu_order,
    wp_posts.post_title asc
LIMIT 0, 10
于 2013-03-15T18:23:15.280 回答
1

两个内连接：

INNER JOIN wp_postmeta ON (wp_posts.ID = wp_postmeta.post_id)
INNER JOIN wp_postmeta AS mt1 ON (wp_posts.ID = mt1.post_id)

代表两个数据集（恰好是“相同的”）。

给定WHERE条件，结果将是两行的集合，其中第一行 ( wp_postmeta) 满足一个条件：

   (wp_postmeta.meta_key = '_visibility' AND CAST(wp_postmeta.meta_value AS CHAR) IN ('visible','catalog'))

第二个满足完全不同的条件：

   (mt1.meta_key = '_stock_status' AND CAST(mt1.meta_value AS CHAR) = 'instock')

通过将所有内容组合为 one INNER JOIN，您正在寻找与这两个条件匹配的 ONE ROW 。显然没有。

如果您查看“出色的解决方案”，您会看到“与”已更改为“或”以保留语义：

AND (
  ( pmta.meta_key = '_visibility' AND CAST(pmta.meta_value AS CHAR) IN ( 'visible','catalog' ) )
  OR ( pmta.meta_key = '_stock_status' AND CAST(pmta.meta_value AS CHAR) = 'instock' )
)
于 2013-03-15T18:28:18.023 回答