OVER语句中的COALESCE函数不起作用

Question

有人可以告诉我为什么COALESCE在这里执行第一个SELECT而不是其他两个吗？我仍然在后两个语句中得到NULL值。

(SELECT COALESCE(DEFax, NULL, '') FROM Debtor d WHERE d.DEIsPrimary = 1 AND d.CApKey = c.CApKey) AS FaxNumberOne,
        (SELECT COALESCE(DEFax, NULL, '') FROM (SELECT ROW_NUMBER() OVER (ORDER BY DEpKey ASC) 
            AS rownumber, DEFax FROM Debtor d WHERE d.CApKey = c.CApKey AND d.DEIsPrimary <> 1) 
            AS foo WHERE rownumber = 1) AS FaxNumberTwo,
        (SELECT COALESCE(DEFax, NULL, '') FROM (SELECT ROW_NUMBER() OVER (ORDER BY DEpKey ASC) 
            AS rownumber, DEFax FROM Debtor d WHERE d.CApKey = c.CApKey AND d.DEIsPrimary <> 1) 
            AS foo WHERE rownumber = 2) AS FaxNumberThree

谢谢！

Answer 1

样本数据和预期结果确实会有所帮助。

但是标量子查询是返回一列和零或一行的子查询。如果返回零行，则无论NULL 中的表达式如何，该值为SELECT 。换句话说，COALESCE()需要走到外面，像这样：

coalesce( (select . . . . ),
          ''
        )

在NULL列表中包含coalesce()不是一个好习惯。这是不必要的且具有误导性-始终被忽略。