Question

以下查询不会返回name1的任何值，因为它没有wp_postmeta.meta_key = 'wpcf-phone'。我正在使用LEFT OUTER JOIN认为它会返回name1的结果，因为它在wp_posts中有值，但它只是name2的返回值，其中wp_postmeta.meta_key = 'wpcf-phone'

SELECT 
  wp_posts.ID,
  wp_posts.post_content,
  wp_posts.post_title,
  wp_postmeta.meta_value AS phone
FROM
  wp_posts
  LEFT OUTER JOIN wp_postmeta wp_postmeta ON (wp_posts.ID = wp_postmeta.post_id)
WHERE
  wp_posts.post_type = 'solicitors' AND 
  wp_posts.post_status = 'publish' AND 
  wp_postmeta.meta_key = 'wpcf-phone' AND 
  (wp_posts.post_title LIKE '%name1%' OR 
  wp_posts.post_title LIKE '%name2%')

Answer 1

您正在此行的where子句中引用外部表：

wp_postmeta.meta_key = 'wpcf-phone'

从而消除wp_postmeta.meta_key为空的所有行，有效地将LEFT JOIN转换为INNER JOIN（因为NULL = 'wpcf-phone'为false）。

您应该将条件移至联接：

SELECT 
  wp_posts.ID,
  wp_posts.post_content,
  wp_posts.post_title,
  wp_postmeta.meta_value AS phone
FROM
  wp_posts
  LEFT OUTER JOIN wp_postmeta wp_postmeta 
    ON (wp_posts.ID = wp_postmeta.post_id)
    AND wp_postmeta.meta_key = 'wpcf-phone' 
WHERE
  wp_posts.post_type = 'solicitors' AND 
  wp_posts.post_status = 'publish' AND 
  (wp_posts.post_title LIKE '%name1%' OR 
  wp_posts.post_title LIKE '%name2%')

Answer 2

当OUTER JOIN'无法连接'时，它会使用NULL值填充输出中的字段。因此，您可以使用OR IS NULL来测试此情况。因此，WHERE子句应该是这样的：

WHERE
  wp_posts.post_type = 'solicitors' AND 
  wp_posts.post_status = 'publish' AND 
  (wp_postmeta.meta_key = 'wpcf-phone' OR
      wp_postmeta.meta_key IS NULL) AND 
  (wp_posts.post_title LIKE '%name1%' OR 
      wp_posts.post_title LIKE '%name2%')

MYSQL LEFT OUTER JOIN查询不返回所有结果

2 个答案: