Question

我正在Microsoft SQL Server中尝试执行以下操作：

DECLARE @isparent int;

SET @isparent = iif(account.parentid is not null,
                    1,
                    0);

SELECT
    IIF(@isparent = 1,
        parentenrollment,
        enrollment)            "Enrollment"
FROM
    Accounts

显然，这被大大简化了，但是其思想是将一个基于表中内容的值存储在变量中（@isparent，上方），然后稍后重新读取该变量，从而确定要显示的值。

这可能吗？如果可以，怎么办？

Answer 1

根据您的代码，您不需要该变量，我认为您正在尝试这样做

SELECT
    IIF(parentid is not null,
        parentenrollment,
        enrollment) Enrollment
FROM
    Accounts

与

相同

SELECT CASE WHEN ParentId IS NOT NULL THEN ParentEnrollment
            ELSE Enrollment
       END Enrollment
FROM Accounts

如果您真的想检查parentid是否为空并将值存储在变量中，则您的查询应返回一（1）行，可以用两种方法完成

TOP和ORDER BY

SELECT TOP 1 @isparent = IIF(parentid IS NOT NULL, 1, 0)
FROM Accounts
ORDER BY <Your Order Here>

WHERE子句，其条件仅返回1行

SELECT @isparent = IIF(parentid IS NOT NULL, 1, 0)
FROM Accounts
WHERE <Type Your Condition(s) Here>

Answer 2

您可以使用：

DECLARE @isparent int;

SELECT @isparent = IIF(parentid is not null, parentenrollment, enrollment)
FROM    Accounts
WHERE ... --  condition that returns single row

Microsoft SQL Server-条件变量分配

2 个答案: