如果左连接返回行，则条件 Where 子句？

Question

我觉得有一种方法可以使用条件 Cases 或 Ifs 来做到这一点，但我就是想不通。

下面的查询将三个表连接在一起，如果所有三个表都返回行，则效果很好。但是，在某些情况下，campaign_groups 和 new_advertisers 中存在记录，但 payment_transactions 中没有。

由于 WHERE 子句确保 payment_transactions.transaction_date >=campaign_groups.date_created，零交易的行被排除在外。

有没有办法只在 LEFT JOIN 返回行时应用 WHERE 子句？

SELECT sum(payment_transactions.transaction_amount) as payment, 
            new_advertisers.account_id, 
            new_advertisers.advertiser_id,
            max(payment_transactions.transaction_date) as last_payment,
            campaign_groups.id,
            campaign_groups.date_created,
            TIMESTAMPDIFF(DAY, max(payment_transactions.transaction_date), NOW()) as days_passed
        FROM campaign_groups
        INNER JOIN new_advertisers ON campaign_groups.advertiser_id = new_advertisers.advertiser_id
        LEFT JOIN payment_transactions ON payment_transactions.account_id = new_advertisers.account_id
        WHERE payment_transactions.transaction_date >= campaign_groups.date_created
            AND campaign_groups.weekly_budget_cents > 0
        ORDER BY days_passed DESC

Answer 1

将条件移到您的 ON 子句中：

LEFT JOIN payment_transactions
    ON payment_transactions.account_id = new_advertisers.account_id
    AND payment_transactions.transaction_date >= campaign_groups.date_created
WHERE campaign_groups.weekly_budget_cents > 0