用多個UNION重寫SQL Server查詢

Question

是否有更好（更高效）的方式來編寫此查詢？似乎應該有一種方法可以只寫一個UNION。

查詢：

SELECT 'Value1' as PropertyKey, PropertyValue=(SELECT Value1 FROM MyTable WITH (NOLOCK)) 
UNION 
SELECT 'Value2' as PropertyKey, PropertyValue=(SELECT Value2 FROM MyTable WITH (NOLOCK)) 
UNION 
SELECT 'Value3' as PropertyKey, PropertyValue=(SELECT Value3 FROM MyTable WITH (NOLOCK)) 
UNION 
... 
SELECT 'Value100' as PropertyKey, PropertyValue=(SELECT Value100 FROM MyTable WITH (NOLOCK)) 

最後，我需要我的結果集有2列（PropertyKey和PropertyValue）。 PropertyKey列中的值將是我表格中列的名稱，而PropertyValue列中的值將是相應的值。

如果我總是從同一張表中選擇，是否可以僅使用一個UNION來編寫此代碼？

Answer 1

如果您有機會獲得UNPIVOT功能，您可以使用它的方式如下：

select PropertyKey, PropertyValue 
from yourtable 
unpivot 
(
    PropertyValue 
    for PropertyKey in (Value1, Value2, Value3) -- list other columns here 
) unpiv 

的關鍵部分要記住與UNPIVOT是數據類型必須是相同的，所以你可能需要轉換數據類型：

select PropertyKey, PropertyValue 
from 
(
    select CAST(value1 as varchar(10)), 
     CAST(value1 as varchar(10)), ... 
    from yourtable 
) src 
unpivot 
(
    PropertyValue 
    for PropertyKey in (Value1, Value2, Value3) -- list other columns here 
) unpiv 

你甚至可以動態地執行此，這將讓列的列表在運行時轉換：

DECLARE @colsUnpivot AS NVARCHAR(MAX), 
    @query AS NVARCHAR(MAX)  

select @colsUnpivot = stuff((select ','+quotename(C.name) 
     from sys.columns as C 
     where C.object_id = object_id('yourtable') 
     for xml path('')), 1, 1, '') 

set @query 
    = 'select PropertyKey, PropertyValue 
    from yourtable 
    unpivot 
    (
     PropertyValue 
     for PropertyKey in ('+ @colsunpivot +') 
    ) u' 

exec(@query) 

如果你被卡住，使用當前版本的話，我會稍微改變，並使用一個UNION ALL代替UNION的：

SELECT 'Value1' as PropertyKey, Value1 PropertyValue 
FROM MyTable 
UNION ALL 
SELECT 'Value2' as PropertyKey, Value2 PropertyValue 
FROM MyTable