MySQL - 计算行和左连接问题

Question

我有2个表,广告系列和campaign_codes:

广告系列:id,partner_id,status

campaign_codes:id,代码,状态

我希望获得所有广告系列的所有广告系列代码的统计信息WHERE campaign_codes.status等于0或者广告系列没有campaign_codes记录.

我有以下SQL,但当然WHERE语句会删除那些在campaign_codes中没有相应记录的广告系列(我希望这些广告系列也包含零广告系列代码)

SELECT 
    c.id AS campaign_id, 
    COUNT(cc.id) AS code_count
FROM 
    campaigns c
LEFT JOIN campaign_codes cc on cc.campaign_id = c.id
WHERE c.partner_id = 4
AND cc.status = 0
GROUP BY c.id

Answer 1

我会选择以下内容:

SELECT 
    c.id AS campaign_id, 
    COUNT(cc.id) AS code_count
FROM 
    campaigns c
LEFT JOIN campaign_codes cc on cc.campaign_id = c.id
AND cc.status = 0 -- Having this clause in the WHERE, effectively makes this an INNER JOIN
WHERE c.partner_id = 4
GROUP BY c.id

移动AND到join子句会使连接成功或失败,关键是将结果行保留在"右"表中没有匹配行的位置.

如果它在WHERE,那么与NULL(没有campaign_code)的比较将失败,并从结果中消除.