左连接中的mysql子查询

2022-01-23 00:00:00 left-join subquery mysql

我有一个查询需要来自名为 tbl_emails_sent 的辅助表中的最新记录.

I have a query that needs the most recent record from a secondary table called tbl_emails_sent.

该表包含发送给客户的所有电子邮件.大多数客户记录了几到数百封电子邮件.我想提取一个显示最新的查询.

That table holds all the emails sent to clients. And most clients have several to hundreds of emails recorded. I want to pull a query that displays the most recent.

例子:

SELECT c.name, c.email, e.datesent
FROM `tbl_customers` c
LEFT JOIN `tbl_emails_sent` e ON c.customerid = e.customerid

我猜会使用带有子查询的 LEFT JOIN，但我并没有深入研究子查询.我的方向正确吗?

I'm guessing a LEFT JOIN with a subquery would be used, but I don't delve into subqueries much. Am I going the right direction?

目前，上述查询并未针对指定表中的最新记录进行优化，因此我需要一些帮助.

Currently the query above isn't optimized for specifying the most recent record in the table, so I need a little assistance.

推荐答案

应该是这样，需要单独查询才能获取邮件发送的最大日期(或最晚日期).

It should be like this, you need to have a separate query to get the maximum date (or the latest date) that the email was sent.

SELECT  a.*, b.*
FROM    tbl_customers a
            INNER JOIN tbl_emails_sent b
                ON a.customerid = b.customerid
            INNER JOIN
            (
                SELECT      customerid, MAX(datesent) maxSent
                FROM        tbl_emails_sent
                GROUP BY    customerid
            ) c ON  c.customerid = b.customerid AND
                    c.maxSent = b.datesent

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