通过MAX搜索SQL()

Question

select max(DELIVERY_TIMESTAMP) from DOCUMENTS;将返回最新文件的交付时间.如何返回最新文档的其他列？例如,我想要DOC_NAME最近交付的文档？

我不确定如何形成这个WHERE条款.

Answer 1

你有几个选择

SELECT DOC_NAME
FROM DOCUMENTS
WHERE DELIVERY_TIMESTAMP IN (
    SELECT MAX(DELIVERY_TIMESTAMP)
    FROM DOCUMENTS
)

或者加入

SELECT DOC_NAME
FROM DOCUMENTS
INNER JOIN (
    SELECT MAX(DELIVERY_TIMESTAMP) AS MAX_DELIVERY_TIMESTAMP
    FROM DOCUMENTS
) AS M
    ON M.MAX_DELIVERY_TIMESTAMP = DOCUMENTS.DELIVERY_TIMESTAMP

如果时间戳中存在重复项,或者您的"最大"条件中需要多列,则会变得更复杂(因为MAX()只有所有行的一列)

这是JOIN选项是唯一可用的选项,因为这样的结构不可用(比如具有相同时间戳的多个订单):

SELECT DOC_NAME
FROM DOCUMENTS
WHERE (DELIVERY_TIMESTAMP, ORDERID) IN (
    SELECT TOP 1 DELIVERY_TIMESTAMP, ORDERID
    FROM DOCUMENTS
    ORDER BY DELIVERY_TIMESTAMP DESC, ORDERID DESC
)

你实际上需要做的事情:

SELECT DOC_NAME
FROM DOCUMENTS
INNER JOIN (
    SELECT TOP 1 DELIVERY_TIMESTAMP, ORDERID
    FROM DOCUMENTS
    ORDER BY DELIVERY_TIMESTAMP DESC, ORDERID DESC
) AS M
    ON M.DELIVERY_TIMESTAMP = DOCUMENTS.DELIVERY_TIMESTAMP
        AND M.ORDERID = DOCUMENTS.ORDERID