即使未满足 WHERE 子句，也从查询返回结果

Question

我正在创建一个查询，以确保满足某些约束。这是一个半工作版本：

SELECT CASE
            WHEN TaskId IS NULL THEN 0
            ELSE 1  
        END AS TaskExists, 
        CASE
            WHEN IsDownTask = 0 AND TaskStatus = 63 THEN 1
            WHEN IsDownTask = 1 THEN 1
            ELSE 0
        END AS PressReady,
        CASE 
            WHEN IsDownTask = 1 AND MachineId <> 2710 THEN 1
            ELSE 0
        END AS DownTaskAssignedToDifferentMachine   
FROM Task T
WHERE TaskId = 555555

当TaskId 存在于Task 表中时，这可以正常工作，但如果该任务不存在（因此TaskExists 字段），我还需要返回值。

对于不存在的任务的查询，我希望返回

• TaskExists 0
• PressReady 0
• DownTaskAssignedToDisfferentMachine 0

即使不存在TaskId，我如何修改我的查询以返回它？

Answer 1

如果您想返回这些值，只需用 SUM 和 ISNULL 包装每一列：

SELECT ISNULL(SUM(CASE
        WHEN TaskId IS NULL THEN 0
        ELSE 1  
    END), 0) AS TaskExists, 
    ISNULL(SUM(CASE
        WHEN IsDownTask = 0 AND TaskStatus = 63 THEN 1
        WHEN IsDownTask = 1 THEN 1
        ELSE 0
    END), 0) AS PressReady,
    ISNULL(SUM(CASE 
        WHEN IsDownTask = 1 AND MachineId <> 2710 THEN 1
        ELSE 0
    END), 0) AS DownTaskAssignedToDifferentMachine    

Answer 2

你可以试试这样的：

DECLARE @task INT
SET @task = 555555

SELECT CASE
            WHEN TaskId IS NULL THEN 0
            ELSE 1  
        END AS TaskExists, 
        CASE
            WHEN IsDownTask = 0 AND TaskStatus = 63 THEN 1
            WHEN IsDownTask = 1 THEN 1
            ELSE 0
        END AS PressReady,
        CASE 
            WHEN IsDownTask = 1 AND MachineId <> 2710 THEN 1
            ELSE 0
        END AS DownTaskAssignedToDifferentMachine   
FROM Task T
WHERE TaskId = @task
UNION ALL 
SELECT 0 TaskExists, 0 PressReady, 0 DownTaskAssignedToDifferentMachine   
WHERE NOT EXISTS (SELECT * FROM  Task WHERE TaskId = @task)

Answer 3

如果它必须在查询中，而不是在代码中，那么只需作弊并将默认行添加到查询的末尾即可。保证默认排序在任何可能的真实结果之后，并限制在您的第一行：

SELECT TOP 1 * FROM (
  SELECT 1 AS TaskExists, CASE ... END AS PressReady, CASE ... AS WowYouHaveALongFieldNameHere
    FROM Task
   WHERE TaskID = 55555

  UNION

  -- default if no matching row
  SELECT 0,               0,                          0
) ORDER BY TaskExists DESC;

您可能会发现这比涉及聚合函数或强制连接和 COALESCE/ISNULLing 等的替代方案更具可读性。

Answer 4

请尝试以下代码。我没有测试过。

SELECT CASE
                WHEN TaskId IS NULL THEN 0
                ELSE 1  
            END AS TaskExists, 
            CASE
                WHEN IsDownTask = 0 AND TaskStatus = 63 THEN 1
                WHEN IsDownTask = 1 THEN 1
                ELSE 0
            END AS PressReady,
            CASE 
                WHEN IsDownTask = 1 AND MachineId <> 2710 THEN 1
                ELSE 0
            END AS DownTaskAssignedToDifferentMachine   
    FROM Task T
    WHERE  1= case when TaskId = 555555 then 1 
    else 0 end