Return result from query even if WHERE clause not met

Question

I am creating a query that ensures some constraints are met. Here's a semi-working version right now:

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

This works fine when TaskId exists in the Task table, but I also need to return values if that Task doesn't exist (hence the TaskExists field).

For a query on a non-existent Task, I'd expect to return

• TaskExists 0
• PressReady 0
• DownTaskAssignedToDisfferentMachine 0

How can I modify my query to return this even when no TaskId exists?

Answer 1

If you want to return those values just wrap each column with a SUM and an ISNULL:

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