SQL CASE checking for two conditions from the same column

Question

I currently have a CASE statement that checks to see whether certain tasks are completed or not, and then returns the date of the next task. Since the tasks are ordered, each WHEN statement becomes longer, checking each of the previous tasks to see if they're complete. For some reason, after the first WHEN statement, it's skipping straight to ELSE (it should be meeting the conditions of the second or third WHEN).

CASE
    WHEN T.PRNAME = 'TECH PEP MEETING DATE' AND T.PRSTATUS != 2 THEN (SELECT TO_CHAR(MAX(T.PRFINISH),'DD/MM/YY') FROM PRTASK T WHERE T.PRPROJECTID = INV_INVESTMENTS.ID AND Upper(T.PRNAME) = 'TECH PEP MEETING DATE' AND PRISMILESTONE = 1)
    WHEN (T.PRNAME = 'TECH PEP MEETING DATE' AND T.PRSTATUS = 2) AND (T.PRNAME = 'BRU MEETING DATE' AND T.PRSTATUS != 2) THEN (SELECT TO_CHAR(MAX(T.PRFINISH),'DD/MM/YY') FROM PRTASK T WHERE T.PRPROJECTID = INV_INVESTMENTS.ID AND Upper(T.PRNAME) = 'BRU MEETING DATE' AND PRISMILESTONE = 1)
    WHEN (T.PRNAME = 'TECH PEP MEETING DATE' AND T.PRSTATUS = 2) AND (T.PRNAME = 'BRU MEETING DATE' AND T.PRSTATUS = 2) AND (T.PRNAME = 'TSC MEETING DATE' AND T.PRSTATUS != 2) THEN (SELECT TO_CHAR(MAX(T.PRFINISH),'DD/MM/YY') FROM PRTASK T WHERE T.PRPROJECTID = INV_INVESTMENTS.ID AND Upper(T.PRNAME) = 'TSC MEETING DATE' AND PRISMILESTONE = 1) 
    ELSE (SELECT TO_CHAR(MAX(T.PRFINISH),'DD/MM/YY') FROM PRTASK T WHERE T.PRPROJECTID = INV_INVESTMENTS.ID AND Upper(T.PRNAME) = 'END OF EVALUATE PHASE' AND PRISMILESTONE = 1)
END

Is there a specific way you need to address these types of multiple WHEN conditions?

EDIT: So after some feedback from you guys I agree with the idea that you only need to evaluate one task per WHEN because the CASE statement should exit once it finds its first TRUE statement. However, having updated it to:

CASE
    WHEN UPPER(T.PRNAME) = 'EVALUATE TECH PEP MEETING DATE' AND T.PRSTATUS != 2 THEN (SELECT TO_CHAR(MAX(T.PRFINISH),'DD/MM/YY') FROM PRTASK T WHERE T.PRPROJECTID = INV_INVESTMENTS.ID AND Upper(T.PRNAME) = 'TECH PEP MEETING DATE' AND PRISMILESTONE = 1 AND ROWNUM = 1)
    WHEN UPPER(T.PRNAME) = 'EVALUATE BRU MEETING DATE' AND T.PRSTATUS != 2 THEN (SELECT TO_CHAR(MAX(T.PRFINISH),'DD/MM/YY') FROM PRTASK T WHERE T.PRPROJECTID = INV_INVESTMENTS.ID AND Upper(T.PRNAME) = 'BRU MEETING DATE' AND PRISMILESTONE = 1 AND ROWNUM = 1)
    WHEN UPPER(T.PRNAME) = 'EVALUATE TSC MEETING DATE' AND T.PRSTATUS != 2 THEN (SELECT TO_CHAR(MAX(T.PRFINISH),'DD/MM/YY') FROM PRTASK T WHERE T.PRPROJECTID = INV_INVESTMENTS.ID AND Upper(T.PRNAME) = 'TSC MEETING DATE' AND PRISMILESTONE = 1 AND ROWNUM = 1) 
    ELSE (SELECT TO_CHAR(MAX(T.PRFINISH),'DD/MM/YY') FROM PRTASK T WHERE T.PRPROJECTID = INV_INVESTMENTS.ID AND Upper(T.PRNAME) = 'END OF EVALUATE PHASE' AND PRISMILESTONE = 1)
END

I am now getting:

ORA-01427: single-row subquery returns more than one row

Not sure why this is the case, especially having put ROWNUM = 1 on the end to ensure only one result is returned.

When running the THEN by itself:

SELECT TO_CHAR(MAX(T.PRFINISH),'DD/MM/YY') 
FROM PRTASK T 
WHERE T.PRPROJECTID = INV_INVESTMENTS.ID 
AND Upper(T.PRNAME) = 'TECH PEP MEETING DATE' 
AND PRISMILESTONE = 1 
AND ROWNUM = 1

I'm getting one result. If I'm right in thinking that the CASE statement will exit once it finds its first TRUE statement, why is this finding multiple rows?

EDIT 2: Ok - so I've been playing around with this some more (because I still cannot find a logical answer and I've made some headway. I have now changed the way the query is structured to the following:

SELECT DISTINCT To_Char(T.PRFINISH, 'DD/MM/YY'),
                T.PRNAME  
FROM  PRTASK T
      LEFT OUTER JOIN INV_INVESTMENTS ON T.PRPROJECTID = INV_INVESTMENTS.ID
WHERE T.PRNAME = CASE 
                     WHEN (T.PRNAME = 'Concept Tech PEP Meeting Date' AND T.PRSTATUS != 2) THEN 'Concept Tech PEP Meeting Date'
                     WHEN (T.PRNAME = 'Concept BRU Meeting Date' AND T.PRSTATUS != 2) THEN 'Concept BRU Meeting Date'
                     WHEN (T.PRNAME = 'End of Concept Phase' AND T.PRSTATUS != 2) THEN 'End of Concept Phase'                                                                                                                
                     WHEN (T.PRNAME = 'Evaluate Tech PEP Meeting Date' AND T.PRSTATUS != 2) THEN 'Evaluate Tech PEP Meeting Date'
                     WHEN (T.PRNAME = 'Evaluate BRU Meeting Date' AND T.PRSTATUS != 2) THEN 'Evaluate BRU Meeting Date'
                     WHEN (T.PRNAME = 'Evaluate TSC Meeting Date' AND T.PRSTATUS != 2) THEN 'Evaluate TSC Meeting Date'
                     WHEN (T.PRNAME = 'End of Evaluate Phase' AND T.PRSTATUS != 2) THEN 'End of Evaluate Phase'                      
                     WHEN (T.PRNAME = 'End of Analyse Phase' AND T.PRSTATUS != 2) THEN 'End of Analyse Phase'
                     WHEN (T.PRNAME = 'End of Design Phase' AND T.PRSTATUS != 2) THEN 'End of Design Phase'
                     WHEN (T.PRNAME = 'End of Build Phase' AND T.PRSTATUS != 2) THEN 'End of Build Phase'
                     WHEN (T.PRNAME = 'End of Test Phase' AND T.PRSTATUS != 2) THEN 'End of Test Phase'
                     WHEN (T.PRNAME = 'In Service' AND T.PRSTATUS != 2) THEN 'In Service'                    
                     WHEN (T.PRNAME = 'End of Implement Phase' AND T.PRSTATUS != 2) THEN 'End of Implement Phase'  
                     WHEN (T.PRNAME = 'End of Closure Phase' AND T.PRSTATUS != 2) THEN 'End of Closure Phase'
                     ELSE 'In Service'
                  END
     AND INV_INVESTMENTS.CODE = '007058'

Now, however, I'm getting multiple WHEN statements returning values. Can anyone confirm whether or not CASE statements truly only return the first TRUE value?

Answer 1

The T.PRNAME = 'TECH PEP MEETING DATE' AND T.PRNAME = 'BRU MEETING DATE'? I think we're having an overlapping conditions here.

My high assumption here is that you have 1 table with many task with status beside, then I think what should happen here is something as below.

First is your table of tasks, I created something my own.

CREATE TABLE #testtask
  (
    PRID INT
    , PRNAME varchar(50)
    , PRSTATUS INT
    , PREREQ INT
  )

  INSERT INTO #testtask VALUES
  (1,'TECH PEP MEETING DATE',1,0),
  (2,'BRU MEETING DATE',1,1),
  (3,'TSC MEETING DATE',1,2)

Must might be something like this

Then, I created a left join on its own table related to its pre-requisite task.

SELECT
     t1.PRNAME AS [Job]
     , t1.PRSTATUS AS [JobStatus]
     , t2.PRNAME AS [PreReqJob]
     , t2.PRSTATUS AS [PreReqStatus]
  INTO #taskList
  FROM #testtask t1
  LEFT JOIN #testtask t2
  ON
    t1.PREREQ = t2.PRID

and with this following result.

before getting into what I believe is your script checking for each task state with pre-requisite tasks.

SELECT
      CASE
        WHEN tl.[Job] = 'TECH PEP MEETING DATE' AND tl.[JobStatus] != 2
            THEN
                -- do your max select here for 'Tech pep'
        WHEN tl.[Job] = 'BRU MEETING DATE' AND tl.[JobStatus] != 2 AND tl.[PreReqStatus] = 2
            THEN
                -- do your max select here for 'Bru meet'
        WHEN tl.[Job] = 'TSC MEETING DATE' AND tl.[JobStatus] != 2 AND tl.[PreReqStatus] = 2
            THEN
                -- do your max select here for 'Tsc meet'
        ELSE
            -- do your default max date
     END AS [Date]
  FROM #taskList AS tl

Please get the concept alone as I do not have your actual tables. You would pretty much have an error if you copy the whole thing. Hopefully this helps :)