Question

我有2个表格，广告系列和广告系列代码：

广告系列：id，partner_id，status

campaign_codes：id，code，status

我想了解所有广告系列的所有广告系列代码的计数WHERE campaign_codes.status等于0或广告系列没有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点广告系列代码）

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

将AND移动到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中，则与NULL（没有campaign_code）的比较会失败，并从结果中删除。

Answer 2

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 c.partner_id = 4
    AND cc.status = 0
GROUP BY c.id

MySQL - 计算行和左连接问题

2 个答案: