SQL 在 LEFT JOIN 中获取 MAX 日期时间

Question

我正在对表运行查询并进行左连接以尝试从左表中获取最新日期的记录，但它没有获取与日期时间列相关的其他值（用户和注释）

SELECT
    i.customer_sequence,
    i.due_date,
    
    MAX(cn.datetime) as notes_datetime,
    cn.user as notes_user,
    cn.notes as notes_notes
FROM
    billing_invoices i
LEFT JOIN customer_notes cn
    ON i.customer_sequence = cn.customer_seq
WHERE
    cn.type = 'Accounts' AND
    i.customer_sequence <> '0' AND
    i.status = 'Unpaid' AND
    i.directdebit <> 'Y'
GROUP BY
    i.customer_sequence
ORDER BY
    i.due_date DESC

Answer 1

聚合不是这里的解决方案。您想要连接表中的整行，因此建议进行过滤。如果您运行的是 MySQL 8.0，我会推荐窗口函数：

SELECT *
FROM (
    SELECT
        i.customer_sequence,
        i.due_date,
        ROW_NUMBER() OVER(PARTITION BY i.customer_sequence ORDER BY cn.datetime DESC) rn,
        cn.datetime as notes_datetime,
        cn.user as notes_user,
        cn.notes as notes_notes
    FROM billing_invoices i
    LEFT JOIN customer_notes cn
        ON  i.customer_sequence = cn.customer_seq
        AND cn.type = 'Accounts'
    WHERE
        i.customer_sequence <> '0' AND
        i.status = 'Unpaid' AND
        i.directdebit <> 'Y'
) t
ORDER BY i.due_date DESC    

请注意，我将left joined 表上的条件从WHERE 子句移至连接的ON 子句（否则，这就像inner join）。

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在早期版本中，一个选项是相关子查询：

SELECT
    i.customer_sequence,
    i.due_date,
    cn.datetime as notes_datetime,
    cn.user as notes_user,
    cn.notes as notes_notes
FROM billing_invoices i
LEFT JOIN customer_notes cn
    ON  i.customer_sequence = cn.customer_seq
    AND cn.type = 'Accounts'
    AND cn.datetime = (
        SELECT MAX(cn1.datetime)
        FROM customer_notes cn1
        WHERE i.customer_sequence = cn1.customer_seq AND cn1.type = 'Accounts'
    )
WHERE
    i.customer_sequence <> '0' AND
    i.status = 'Unpaid' AND
    i.directdebit <> 'Y'