sql - 交叉应用与 UNPIVOT

Question

我发现 UNPIVOT 能够自动排除具有 NULL 值的字段。但是，CROSS APPLY-VALUES 方法无法做到这一点。有谁知道如何从 CROSS APPLY-VALUES 中自动排除 NULL 值字段？比方说，如果字段 Field3 和 Field4 包含 NULL 值，则排除它们。

SELECT 
    E.FieldA, 
    E.FieldB, 
    E.FieldC, 
    DBParam.Display, 
    DBParam.Value
INTO DBParam
FROM
Map_Data AS E
CROSS APPLY (VALUES (Field1, 'Field1'),
            (Field2, 'Field2'), 
            (Field3, 'Field3'),
            (Field4, 'Field4')
) AS DBParam(Value, Display)

Answer 1

不要使用 VALUES，而是使用 SELECT:

SELECT E.FieldA, E.FieldB, E.FieldC, 
       DBParam.Display, DBParam.Value
INTO DBParam
FROM Map_Data E CROSS APPLY
     (SELECT Value, Display
      FROM (SELECT Field1 as Value, 'Field1' as Display UNION ALL
            SELECT Field2, 'Field2' UNION ALL
            SELECT Field3, 'Field3' UNION ALL
            SELECT Field4, 'Field4'
           ) vd
      WHERE Value is not null
     ) as DBParam(Value, Display);