MYSQL：仅左连接第一行或选择不同的

Question

我正在尝试在几个表上进行左连接。但是，它显示了所有左连接表结果。主表将是“a”，但它显示重复的 id，因为它的 id 在其他表上多次存在。如何只选择其他表格的第一行？或选择表“a”上的不同结果？这是我的 SQL

SELECT a.adsId, a.adsName, a.adsUrl, a.sourceFileName, c.userName, d.preferenceId, e.ppv FROM y5ads_new.ads_source a 
                INNER JOIN y5ads_new.campaign b ON a.campaignId=b.campaignId 
                INNER JOIN y5ads_new.system_member c ON b.createdBy=c.memberId
                INNER JOIN y5ads_new.ads_preference d ON a.adsId=d.adsId
                INNER JOIN y5ads_new.ads_rates e ON a.adsId=e.adsId
                WHERE a.status='2' AND a.deleted='False' AND CURDATE() BETWEEN a.startDate and a.endDate 
                AND b.status='2' AND b.deleted='False' AND CURDATE() BETWEEN b.startDate and b.endDate 
                AND d.deleted='False' AND CURDATE() BETWEEN d.startDate and d.endDate 
                AND e.is_active='1' AND CURDATE() BETWEEN e.startDate and e.endDate 
                ORDER BY a.adsId DESC, d.preferenceId DESC, e.id DESC

Answer 1

使您的 ID 与众不同并使用 LEFT JOIN：

SELECT a.adsId, a.adsName, a.adsUrl, a.sourceFileName, c.userName, d.preferenceId, e.ppv 
FROM y5ads_new.ads_source AS a 
LEFT JOIN (SELECT DISTINCT campaignId as campaignId FROM y5ads_new.campaign) AS b 
ON a.campaignId=b.campaignId 
LEFT JOIN (SELECT DISTINCT memberId AS memberId, userName FROM y5ads_new.system_member) AS c 
ON b.createdBy=c.memberId
LEFT JOIN (SELECT DISTINCT adsId AS adsId, preferenceId FROM y5ads_new.ads_preference) AS d 
ON a.adsId=d.adsId
LEFT JOIN (SELECT DISTINCT adsId AS adsId, ppv FROM y5ads_new.ads_rates) AS e 
ON a.adsId=e.adsId
WHERE ...