wordpress - MYSQL 菜鸟请教下这个长语句的意思 | 关键词 : LEFT JOIN ON ...

Question

DELETE a,b,c FROM wp_posts a
LEFT JOIN wp_term_relationships b ON (a.ID = b.object_id)
LEFT JOIN wp_postmeta c ON (a.ID = c.post_id)
WHERE a.post_type = 'revision'

这是 WP 用来删除 删除修订版 Revision 的语句, 不明白 LEFT JOIN 是什么个意思 ?

Answer 1

mysql JOIN 默认INNER JOIN 内联 当左右条件均满足
LEFT JOIN 外联 以左表为基础 右表条件不满足的字段返回NULL
RIGHT JOIN 外联 以右表为基础 左表不满足的字段返回NULL