Question

我想确定哪些表包含最早的记录。要做到这一点，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”附近有语法错误。

有什么想法吗？

Answer 1

您需要为子查询提供别名：

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

您尚未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

为了识别子查询，需要

ALIAS。

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

所有其他的（到目前为止）都有正确的答案（你需要派生表的别名），但我也建议你不要使用UNIONing并对所有的CreateDate和TableName值进行排序，而只需要最小的CreateDate从每个表中，当你不需要消除重复时，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

从UNION Subquery获得前1名

4 个答案: