SQL IF Statement to Insert Into the same table - sql

Question

I have a need to query a data set that is stored in two different SQL schemas depending on how we receive the data. I have created the two queries to look at dataset 1 and dataset 2 and it formats that data into two separate temp tables (in the same format).
I'm now trying to do the second step which is working out which temp table has been populated putting this data into a new temp table in order to move on to step 3.
To do this, I'm trying to create an IF statement to work out which dataset we receive but can't seem to get the query to work (even though I have seen solutions from other people's similar queries).
This is my code:
IF object_id('tempdb..#Final_Prem') IS NOT NULL
BEGIN
DROP TABLE #Final_Prem
END
DECLARE #Command varchar(500)
DECLARE #DS1 AS FLOAT
DECLARE #DS2 AS FLOAT
SET #DS1 = (SELECT SUM(PREM) FROM #Dataset1 )
SET #DS2 = (SELECT SUM(PREM) FROM #Dataset2 )
IF (#DS1 IS NULL OR #DS1 = 0)
BEGIN
SET #Command = 'SELECT * INTO #Final_Prem FROM #Dataset2'
END ELSE BEGIN
SET #Command = 'SELECT * INTO #Final_Prem FROM #Dataset1'
END
EXECUTE (#Command)
SELECT * FROM #Final_Prem
The error I keep getting is
Msg 208, Level 16, State 0, Line 18
Invalid object name '#Final_Prem'
Any help would be much appreciated, thanks

Answer 1

You cannot use single-hash tables in dynamic queries. The session that runs the dynamic query is different than the outside one, and thus has no access to it.
If you are sure you will be running the proc/query once each time, you can change the # table to a ##table.
If you want to use both dynamic sql and concurrent runnings, you can save ##SPID into a variable and use it on the end of the ## table names.

Answer 2

The query as a whole needs to compile before any of it can run, and it looks like the last line SELECT * FROM #Final_Prem is causing compilation to fail because the #Final_Prem table does not (yet) exist at that stage.
If you are using SSMS, then you can fix it by putting a line containing only GO after the EXECUTE statement. GO is a pseudo statement (not part of the SQL language) that chops the input into batches( pieces), which are compiled and run one by one, thus preventing such compilation issues.

Answer 3

You're having a scope issue.
A temporary table is only visible to the session where it's created. Your table #Final_Prem is created within the scope of the EXECUTE (#Command) statement, which, unfortunately, is it's own session, and is different from the scope of the calling procedure. As soon as that EXECUTE() is done, the temp table is dropped.
You have two options.
Create a real table.
Use a global temporary table (##Final_Prem instead of
#Final_Prem).
Either way, the table will be accessible from both the inner and outer sessions.
Edit (response to comment): Code in comments is pretty hard to read, so here's what the code would look like using a global temp table:
IF object_id('tempdb..##Final_Prem') IS NOT NULL BEGIN DROP TABLE ##Final_Prem END
DECLARE #Command varchar(500)
DECLARE #DS1 AS FLOAT
DECLARE #DS2 AS FLOAT
SET #DS1 = (SELECT SUM(PREM) FROM ##Dataset1 )
SET #DS2 = (SELECT SUM(PREM) FROM ##Dataset2 )
IF (#DS1 IS NULL OR #DS1 = 0)
BEGIN
SET #Command = 'SELECT * INTO ##Final_Prem FROM ##Dataset2'
END ELSE BEGIN3
SET #Command = 'SELECT * INTO ##Final_Prem FROM ##Dataset1'
END
EXECUTE (#Command)
SELECT * FROM ##Final_Prem
I also changed the two Dataset tables to globals, because they're both accessed from both sessions, too. So you'd need to change them elsewhere, since they're not defined within this code block.

Answer 4

You can simply place the select statement inside the dynamic SQL too like below:
IF (#DS1 IS NULL OR #DS1 = 0)
BEGIN
SET #Command = 'SELECT * INTO #Final_Prem FROM #Dataset2 SELECT * FROM #Final_Prem'
END ELSE BEGIN
SET #Command = 'SELECT * INTO #Final_Prem FROM #Dataset1 SELECT * FROM #Final_Prem'
END
EXECUTE (#Command)
Please check the db<>fiddle here.