按长度限制下划线组

Question

我有很长的文章列表，我按类别分组，如何限制它，以便每个类别只有4个（或任何其他数字）项目？

我的意见是：

我希望每个部分只有4篇文章

Answer 1

将对象值映射到自己的切片版本：

SELECT p1.*, wm2.meta_value 
FROM wp_posts p1 
LEFT JOIN wp_postmeta wm1 ON (
    wm1.post_id = p1.id
    AND wm1.meta_value IS NOT NULL
    AND wm1.meta_key = '_thumbnail_id'
)
LEFT JOIN
wp_postmeta wm2
ON (
    wm1.meta_value = wm2.post_id
    AND wm2.meta_key = '_wp_attached_file'
    AND wm2.meta_value IS NOT NULL
)
LEFT JOIN
wp_term_relationships wtr
ON (
    object_id=p1.id
)
WHERE
p1.post_status='publish'
AND p1.post_type='post'
AND object_id NOT IN (SELECT `object_id` FROM wp_term_relationships where `term_taxonomy_id`=81)
GROUP BY ID
ORDER BY p1.post_date DESC
LIMIT 0,10