Question

How to convert this T-SQL query into Oracle?

if((select case 
       when (select  top 1 AlertMessage   
             from ims.dbo.alertlist 
             where SystemID=@meter 
             order by TimePeriod desc
            ) like '%NO_COMMUNICATION_Resolved' then 1 
       else 0 end)=1)
begin
     INSERT INTO [ims].[dbo].[alertlist]
          ([SiteID],[ThresholdNumber],[SystemID],
           [AlertMessage],[TimePeriod],[AlertType],[PollID]) 
     VALUES 
          (1,@thresnumber,@meter,@message,getdate(),1,0)
end

Answer 1

As pointed out by Dan Puzey, your question is too broad and it would require a lot of trial and error. However, I’ll try to put you on the right track with something that might solve the first part of your problem.

DECLARE v_IsMessageResolved int;

-- This query will retrieve the value of the first row returned by the sub-query, mimicking the TOP 1 of SQL Server, and it will store the value of MessageResolved in a Variable
SELECT
  MessageResolved into v_IsMessageResolved
FROM
  (
  -- This query will return 1 for all the Messages that match the LIKE clause, ordered by TimePeriod in descending order.
  SELECT
    CASE
      WHEN AlertMessage LIKE '%NO_COMMUNICATION_Resolved' THEN 1
      ELSE 0
    END AS MessageResolved
    ,RANK() OVER (ORDER BY TimePeriod DESC) AS MessageRank
  FROM
    ims.alertlist 
  WHERE
    (SystemID = :meter)
  )
WHERE
  (MessageRank = 1)

-- At this point, it will be a matter of checking the value of v_IsMessageResolved and, if it's "1", run the INSERT

Please note that I know SQL Server very well, but I never used Oracle, therefore my solution might not be perfect (or even run at all, as I don’t have an environment to test it). This also means that you can find the answers to the remaining questions you might have with a simple search, as I did. 🙂