来自多个匹配的查询的UPDATE行

Question

给出类似的更新声明：

UPDATE
     UserAssesment
SET
     AssessmentDate = comp.AssessmentDate     
FROM
     UserAssesment ua
INNER JOIN 
    vw_CompletedAssessments comp
On 
    ua.NatId = comp.NatId and
    ua.FamilyName = comp.ClientLastName and
    ua.GivenName = comp.ClientFirstName

WHERE
     ua.HasCompletedAssessment <> 0

如果用户可以有多条匹配join子句的记录到vw_CompletedAssessments，哪条记录将用于更新？有没有办法订购它以便使用最大或最小AssessmentDate？

Answer 1

你的UPDATE语法需要一些调整，见下文：

UPDATE ua
SET
     ua.AssessmentDate = comp.AssessmentDate     
FROM  UserAssesment ua
INNER JOIN  vw_CompletedAssessments comp
ON  ua.NatId = comp.NatId and
    ua.FamilyName = comp.ClientLastName and
    ua.GivenName = comp.ClientFirstName
WHERE ua.HasCompletedAssessment <> 0

现在，如果您有多个值，并且想要从Comp表中选择一个特定值，那么您可以使用ROW_NUMBER函数，就像这样......

UPDATE ua
SET
     ua.AssessmentDate = comp.AssessmentDate     
FROM  UserAssesment ua
INNER JOIN  (SELECT * 
            , ROW_NUMBER() OVER (PARTITION BY NatId ORDER BY AssessmentDate DESC) rn 
            FROM vw_CompletedAssessments) comp
ON  ua.NatId      = comp.NatId 
and ua.FamilyName = comp.ClientLastName 
and ua.GivenName  = comp.ClientFirstName
WHERE ua.HasCompletedAssessment <> 0
AND Comp.rn = 1

此查询会将ua.AssessmentDate更新为特定NatId的最新comp.AssessmentDate。同样，您可以看到如何使用行号操作结果。如果要将其更新为最旧的comp.AssessmentDate值，只需将row_number（）函数中的order by子句更改为ASC，依此类推....

Answer 2

如果找到多个匹配项，则会更新所有匹配项。

如果您只想更新一个，可以使用UPDATE TOP（1）吗？

如果您想保证要更新的订单的顺序，请尝试添加适当的ORDER BY子句。