动态添加列查询

Question

我想创建一个动态查询，根据错误代码选择一个特定的列。

，比如我有一个错误表（ErrorTable），其中包含多个列：

Transaction Number 
Transaction Amount 
Transaction Date 
Error Code 
Error Description 
+100 other columns 

错误表

Error Code 
Error Description 
Error Column 

我想获得一个查询，会得到如下：

ErrorCode 
ErrorDescription 
ErrorColumn (Column Based On Error Code) 

我尝试使用如下的动态SQL，仍然只有c的名称olumn回来了，也许我做错了什么？

DECLARE @SQL VarChar(1000) 
SELECT @SQL = 'SELECT et.ErrorCode, et.ErrorDescription, e.ErrorColumn 
       FROM ErrorTable et 
        INNER JOIN Errors e ON e.ErrorCodeID = et.ErrorCode' 
Exec (@SQL) 

如果我使用@ErrorColumn =代替e.ErrorColumn“TransactionDate”的动态查询我得到的结果，但与上面的查询我不知道。有任何想法吗？

更新2

我得到上面的查询结果如下：

ErrorCode ErrorDesc    TransactionDate TransactionAmount 
1   Invalid Trans Date  TransactionDate TransactionAmount 
2   Invalid Trans Amount TransactionDate TransactionAmount 

我想以下几点：

ErrorCode ErrorDesc    TransactionDate TransactionAmount 
1   Invalid Trans Date  May 1st    
2   Invalid Trans Amount      65 Cats 

Answer 1

你的意思要做到这一点：

DECLARE @ErrorColumn SYSNAME = N'TransactionDate'; 
-- presumably the above is a parameter to the procedure 

DECLARE @sql NVARCHAR(MAX); 

SELECT @sql = N'SELECT et.ErrorCode, et.ErrorDescription, et.' 
    + QUOTENAME(@ErrorColumn) 
    + ' FROM dbo.ErrorTable AS et 
     INNER JOIN dbo.Errors AS e 
     ON e.ErrorCodeID = et.ErrorCode;'; 

EXEC sp_executesql @sql; 

基础上，进一步的信息，也许你想要的是这样的：

SELECT et.ErrorCode, et.ErrorDescription, 
    TransactionDate = CASE et.ErrorColumn WHEN N'TransactionDate' 
    THEN e.TransactionDate ELSE NULL END, 
    TransactionAmount = CASE et.ErrorColumn WHEN N'TransactionAmount' 
    THEN e.TransactionAmount ELSE NULL END 
FROM dbo.ErrorTable AS et 
INNER JOIN dbo.Errors AS e 
ON et.ErrorCode = e.ErrorCodeID; 

或者这样：

SELECT et.ErrorCode, et.ErrorDescription, 
    TransactionDate = CASE et.ErrorColumn WHEN N'TransactionDate' 
    THEN e.TransactionDate ELSE '' END, 
    TransactionAmount = CASE et.ErrorColumn WHEN N'TransactionAmount' 
    THEN e.TransactionAmount ELSE '' END 
FROM dbo.ErrorTable AS et 
INNER JOIN dbo.Errors AS e 
ON et.ErrorCode = e.ErrorCodeID; 

如果需要只返回实际存在的数据集列，它是略更令人费解。您基本上必须构建动态SQL以仅包含最终查询中必须引用的列。

DECLARE @sql NVARCHAR(MAX) = N''; 

SELECT @sql += ',' + CHAR(13) + CHAR(10) 
    + et.ErrorColumn + ' = CASE et.ErrorColumn WHEN N''' 
    + et.ErrorColumn + ''' THEN e.' + et.ErrorColumn + ' ELSE NULL END' 
FROM dbo.ErrorTable AS et INNER JOIN dbo.Errors AS e 
ON et.ErrorCode = e.ErrorCodeID 
GROUP BY et.ErrorColumn; 

SELECT @sql = N'SELECT et.ErrorCode, et.ErrorDescription' 
    + @sql + ' 
    FROM dbo.ErrorTable AS et 
INNER JOIN dbo.Errors AS e 
ON et.ErrorCode = e.ErrorCodeID;'; 

PRINT @sql; 

-- EXEC sp_executesql @sql;