Case Statement in IF-Statement

Are you looking for the below query. You can not use IF within SELECT statement.

SELECT WFO.OrderID, WFO.OrderNumber, 
CASE WHEN (WFO.OrderNumber) > 0 THEN 
        CASE 
        WHEN EXISTS (SELECT *  FROM WF_Job AS WFJ WHERE WFJ.OrderID = WFO.OrderID AND (WFJ.JobsDeliStatusFlag <> '1' OR  WFJ.JobsDeliStatusFlag IS NULL OR WFJ.JobsDeliStatusFlag=' ' )) 
                 AND WFO.OrderID NOT IN (SELECT OrderID FROM DeliveryAccess)
                 THEN '2' 
        WHEN EXISTS (SELECT *  FROM WF_Job AS WFJ WHERE WFJ.OrderID = WFO.OrderID AND WFJ.JobsDeliStatusFlag = '1' AND ISNULL(WFJ.JobsInvStatusFlag, '0') <> '1') 
                 AND WFO.OrderID IN (SELECT OrderID FROM DeliveryAccess)
                 THEN '3' 
        WHEN EXISTS (SELECT *  FROM WF_Job AS WFJ WHERE WFJ.OrderID = WFO.OrderID AND WFJ.JobsDeliStatusFlag = '1' AND WFJ.JobsInvStatusFlag = '1') 
                 AND WFO.OrderID IN (SELECT OrderID FROM DeliveryAccess)
                 THEN '4' 
        END 
ELSE 1 
END AS OrderStatus 
FROM  dbo.WF_Order AS WFO 

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Regards, RSingh