Question

我有一个内部带有子查询的选择

SELECT
....
 , (SELECT TOP 1
                        [DDC].[ContractedAmount]
                        FROM @CustomersTable AS [DDC]
                        WHERE [DDC].[DesignKey] = [D].[DesignKey]) AS [Contracted Amount]
     FROM....

但是有时候这个选择的值可以是0.00，所以我想添加条件值，如果它是0.00，则只返回NULL，所以我做一个IIF条件条件，如：

     , IIF((SELECT TOP 1
                            [DDC].[ContractedAmount]
                            FROM @CustomersTable AS [DDC]
                            WHERE [DDC].[DesignKey] = [D].[DesignKey]) = 
                            0.00,
                            NULL,
           (SELECT TOP 1
                            [DDC].[ContractedAmount]
                            FROM @CustomersTable AS [DDC]
                            WHERE [DDC].[DesignKey] = [D].[DesignKey])) AS [Contracted Amount]

但是正如您所看到的，我需要在子查询中执行另一个子查询，所以我认为这不是实现该目标的最佳实践吗？还有另一种方法可以使此操作更容易阅读。有什么建议么？问候

Answer 1

您可以使用NULLIF：

如果两个指定的表达式相等，则返回空值。

SELECT ...
  NULLIF((subquery), 0.00) AS [Contracted Amount]

另一种方法是使用CROSS/OUTER APPLY：

SELECT ...,
    IIF(sub.ContractedAmount = 0.00, NULL, sub.ContractedAmount) AS [Contracted Amount]
FROM tab
OUTER APPLY (SELECT TOP 1 [DDC].[ContractedAmount]
            FROM @CustomersTable AS [DDC]
            WHERE [DDC].[DesignKey] = [D].[DesignKey])) sub

Answer 2

只需向子查询添加where子句：

(SELECT TOP 1 [DDC].[ContractedAmount]
 FROM @CustomersTable AS [DDC]
 WHERE [DDC].[DesignKey] = [D].[DesignKey] AND
       [DDC].[ContractedAmount] <> 0
) AS [Contracted Amount]

如果未返回任何行，则外部查询中的值为NULL。

使用子查询进行IIF条件的最佳方法

2 个答案: