从UNION子查询中获得排名第一

Question

我想确定哪些表中包含最早的记录。要做到这一点，我想我可能只是说：

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它告诉我那里有一个语法错误附近“秩序”。

任何想法？

Answer 1

你需要给你的子查询的别名：

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

你没有在子查询定义一个别名，

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需要为了要识别的子查询。

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

所有的人（到目前为止）有正确答案（您需要为您的派生表的别名），但我还建议您不要将所有的CreateDate和TableName值进行UNIONing和排序，而是从每个表中获取最小的CreateDate，并培养在不需要消除重复项时使用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