tsql - 从 UNION 子查询中获得前 1

Question

我想确定哪些表中包含最早的记录。为此，我想我可以说：

SELECT TOP 1 TableName FROM
( 
  SELECT CreateDate, 'Table1' as TableName FROM Table1 
  UNION
  SELECT CreateDate, 'Table2' as TableName FROM Table2
)
ORDER BY CreateDate

在 SQL Server 2008R2 中，它告诉我“ORDER”附近存在语法错误。

有任何想法吗？

Answer 1

你需要给你的子查询一个别名：

SELECT TOP 1 TableName FROM
( 
  SELECT CreateDate, 'Table1' as TableName FROM Table1 
  UNION
  SELECT CreateDate, 'Table2' as TableName FROM Table2
) q
ORDER BY CreateDate

Answer 2

您还没有在子查询上定义别名，

SELECT TOP 1 TableName 
FROM
     ( 
         SELECT CreateDate, 'Table1' as TableName FROM Table1 
         UNION
         SELECT CreateDate, 'Table2' as TableName FROM Table2
     ) aliasName     -- <<== ADD HERE
ORDER BY CreateDate

ALIAS为了识别子查询是必需的。

Answer 3

SELECT TOP 1 TableName FROM
( 
  SELECT CreateDate, 'Table1' as TableName FROM Table1 
  UNION
  SELECT CreateDate, 'Table2' as TableName FROM Table2
) RandomName
ORDER BY CreateDate

Answer 4

所有其他人（到目前为止）都有正确的答案（您需要派生表的别名），但我也建议不要对所有 CreateDate 和 TableName 值进行 UNIONing 和排序，而是从每个表中获得最小的 CreateDate ，并在不需要消除重复时养成使用UNION ALL的习惯。所以，像这样：

SELECT TOP 1 TableName FROM
( 
  SELECT MIN(CreateDate) AS CreateDate, 'Table1' as TableName FROM Table1 
  UNION ALL
  SELECT MIN(CreateDate) AS CreateDate, 'Table2' as TableName FROM Table2
) x
ORDER BY CreateDate ASC