SQL 更新第一个记录实例 - SQL 服务器

Question

我有一个导入过程需要更新 table 中旧记录的 "End Date"。导入来自其他人，缺少我需要匹配特定用户信息的一些详细信息。我需要做的是更新旧记录，在下面的示例中，它将是 UserID = 1，而不是更新 UserID = 4.

我有以下 sql 要更新，但如您所知，它更新了两条记录：

UPDATE t1
SET t1.[EndDate] = t2.[EndDate]
FROM ExistingUser AS t1, ImportUser AS t2
WHERE (t2.[uName] = t1.[uName]) AND (t1.[EndDate] IS NULL);

免责声明：我没有创建数据库，无法重新设计tables，所以请可怜我。谢谢！！！

现有用户 - Table

UserID    uName   BeginDate    EndDate
  1       John   01/01/2013
  2       Mary   05/01/2014   04/30/2015
  3       Bob    12/01/2014
  4       John   06/01/2015

导入用户 - Table

uName    EndDate
John    05/31/2015

Answer 1

这可以通过构建一个子查询来完成，该子查询按没有填充 EndDate 的用户名对条目进行编号。这个子查询看起来像这样：

SELECT [UserID], [uName],
  ROW_NUMBER() OVER (PARTITION BY [uName] ORDER BY [BeginDate]) AS IX
FROM ExistingUser
WHERE [EndDate] IS NULL

演示：http://www.sqlfiddle.com/#!6/72c49/2

| UserID | uName | IX |
|--------|-------|----|
|      3 |   Bob |  1 |
|      1 |  John |  1 |
|      4 |  John |  2 |

在该子查询中，John的uName相同的两条记录按BeginDate顺序编号。现在，如果我们将原始 UPDATE 查询加入到此子查询中以 IX=1 搜索记录，我们将只更新最早的行。

WITH EarliestExistingUser AS (
  SELECT [UserID], [uName],
    ROW_NUMBER() OVER (PARTITION BY [uName] ORDER BY [BeginDate]) AS IX
  FROM ExistingUser
  WHERE [EndDate] IS NULL
)
UPDATE t1
SET t1.[EndDate] = t2.[EndDate]
FROM ExistingUser AS t1
  JOIN EarliestExistingUser AS t1f ON t1.UserID = t1f.UserID
,ImportUser AS t2
WHERE (t2.[uName] = t1.[uName]) AND (t1.[EndDate] IS NULL)
AND (t1f.IX = 1)

演示：http://www.sqlfiddle.com/#!6/72c49/3

SQL 更新第一个记录实例 - SQL 服务器

SQL Update first instance of record - SQL Server

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