Question

如果查询没有返回结果，我可以设置变量吗？

DECLARE @Output AS VARCHAR(MAX);

SELECT @Output = 'Old';
SELECT @Output = 'New' WHERE 0 = 1;

PRINT @Output;

输出值为：旧

预期值为： NULL

Answer 1

"inputDescriptors": [
     {
       "id": "connection",
       "name": "Artifactory service",
       "inputMode": "combo",
       "isConfidential": false,
       "hasDynamicValueInformation": true,
       "validation": {
            "isRequired": true,
            "dataType": "string",
            "maxLength": 300
       }
     },
     {
       "id": "definition",
       "name": "definition",
       "description": "Name of the build.",
       "inputMode": "combo",
       "isConfidential": false,
       "dependencyInputIds": [
             "connection"
       ],
       "validation": {
            "isRequired": true,
            "dataType": "string",
            "maxLength": 300
        }
      },
      {
        "id": "buildNumber",
        "name": "Build Number",
        "description": "Number of the build.",
        "inputMode": "combo",
        "isConfidential": false,
        "dependencyInputIds": [
            "connection"
         ],
        "validation": {
             "isRequired": true,
             "dataType": "string",
             "maxLength": 300
        }
       }
],
"dataSourceBindings": [
   {
       "target": "definition",
       "dataSourceName": "BuildNames",
       "resultTemplate": "{ Value : \"{{uri}}\", DisplayValue : \"{{uri}}\" }"
    },
    {
       "target": "versions",
       "dataSourceName": "BuildNumbers",
       "resultTemplate": "{ Value : \"{{uri}}\", DisplayValue : \"{{uri}}\" }"
    },
    {
       "target": "latestVersion",
       "dataSourceName": "BuildNumbers",
       "resultTemplate": "{ Value : \"{{uri}}\", DisplayValue : \"{{uri}}\" }"
    },
    {
       "target": "artifactDetails",
       "resultTemplate": "{ Name: \"{{version}}\", downloadUrl : \"{{endpoint.url}}\" }"
    },
    {
       "target": "buildNumber",
       "dataSourceName": "BuildNumbers",
       "resultTemplate": "{ Value : \"{{uri}}\", DisplayValue : \"{{uri}}\" }"
    }
  ]
 }

您得到DECLARE @Output AS VARCHAR(MAX); SELECT @Output = 'Old'; SELECT @Output = 'New' WHERE 0 = 1; PRINT @Output;，因为变量已经具有该值，并且不会更新该值，因为在'Old'子句中，您使用条件WHERE，该条件将返回{{1 }}，并且变量中的值不会更改。

0=1将是错误的

FALSE将为真

它与WHERE 0 = 1

类似

因此，在您的情况下，值将始终为WHERE 0 <> 1，也不会返回IF 0=1 THEN UpdateMyVar或'Old'。

我不知道您实际上打算做什么，但是如果您想返回'New'，那么

NULL

Answer 2

尝试一下：

DECLARE @Output AS VARCHAR(MAX);
SELECT @Output = 'Old';
SELECT @Output = CASE 0 WHEN 1 THEN 'New' ELSE NULL END
SELECT @Output;

Answer 3

萨米人对发生的事情提供了很好的解释。

如果要确保已分配值，则需要确保查询返回该分配的一行。一种方法是使用聚合：

DECLARE @Output AS VARCHAR(MAX);
SELECT @Output = 'Old';
SELECT @Output = MAX('New') WHERE 0 = 1;
SELECT @Output;

Answer 4

这就是在SQL Server中实现在SELECT中分配变量的方式。正如@SMor在评论中所说，请使用SET而不是SELECT。

DECLARE @Output AS VARCHAR(MAX);

SET @Output = 'Old';
SET @Output = (SELECT 'New' WHERE 0 = 1);
-- this would change the @Output value to NULL

SELECT @Output;

SET的唯一问题是它只能分配一个变量，而在SELECT中可以同时分配多个变量。

SQL Server选择变量，没有结果

4 个答案: