我从这个wordpress查询中得到了双打：
SELECT LEFT(main.post_title , 1) AS `character`,
main.post_title AS title,
main.post_content AS content,
parent_data.name AS parent_title,
root_data.name AS root_title,
thumbnail_data.meta_value AS thumbnail_url,
main.ID
FROM wp_jlwx_posts AS main
INNER JOIN wp_jlwx_term_relationships AS parent ON main.ID = parent.object_id
INNER JOIN wp_jlwx_term_taxonomy AS parent_taxonomy ON parent.term_taxonomy_id = parent_taxonomy.term_taxonomy_id
INNER JOIN wp_jlwx_terms AS parent_data ON parent_taxonomy.term_id = parent_data.term_id
INNER JOIN wp_jlwx_term_taxonomy AS root_taxonomy ON parent_taxonomy.parent = root_taxonomy.term_taxonomy_id
INNER JOIN wp_jlwx_terms AS root_data ON root_taxonomy.term_taxonomy_id = root_data.term_id
LEFT JOIN wp_jlwx_postmeta AS linker ON main.ID = linker.post_id AND linker.meta_key = '_thumbnail_id'
LEFT JOIN wp_jlwx_postmeta AS thumbnail_data ON linker.meta_value = thumbnail_data.post_id
WHERE main.post_type = 'business'
AND main.post_status = 'publish'
$search_where_clause
$character_have_clause
ORDER BY root_title, parent_title, title
我跳出的第一件事是我在分类表上加入了两次。如何确保"主"表中的每一行只返回一行？
（编辑）我能够将其缩小到缩略图连接，我在其中从元表中接收额外的数据：例如 a：5：{s：5："宽度";i：960;s：6："高度"...
我
能够找到问题：
Wordpress 在将缩略图添加到元表中时存储元数据。
该位置与meta_key：一起存储，_wp_attached_file额外的数据存储在meta_key：_wp_attachment_metadata中。
因此，如果我将meta_key ="_wp_attached_file"添加到最后一个连接中，我的问题就解决了。