为什么他们在此代码中使用左连接而不是内连接

Question

我是sql中的新手......这个查询似乎与内部联接运行完全正常，然后为什么作者仍然选择了左连接？

SELECT * 
FROM {$wpdb->posts} notices

LEFT JOIN {$wpdb->postmeta} instances 
ON ( notices.ID = instances.post_id AND instances.meta_key = 'mycred_email_instance' )

LEFT JOIN {$wpdb->postmeta} pointtype 
ON ( notices.ID = pointtype.post_id AND pointtype.meta_key = 'mycred_email_ctype' )

WHERE instances.meta_value = %s 
AND pointtype.meta_value IN (%s,'all') 
AND notices.post_type = 'mycred_email_notice' 
AND notices.post_status = 'publish'

提前致谢！