For how long does a temporary table "live" in dynamic SQL code in Msn SSMS? - sql

Question

I have a problem with temporary tables that does not "live" long enough in my code.
My problem looks like this: I want to create a temporary table in one "codevariable" and use it later. An example of my code structure is like below:
declare #RW varchar(MAX)
set #RW = '
select *
into #temptable
from table1'
exec(#RW)
--Alots of other code.
select *
from #temptable
This results in an error message that sais "Invalid object name '#temptable'. And it's very clear that my temporary table does'nt exists anymore. But I've checked that the table creates in the first step. For example the following code works:
declare #RW varchar(MAX)
set #RW = '
select *
into #temptable
from table1
select *
from #temptable'
exec(#RW)
So my GUESS is that the temporary table only lives within it's code variable. Is there a way to create a temporary table that lives longer? Or, do I just needs to accept this for what it is or am I missing something? I have a work around solution that is not very efficient. What I'm thinking of is creating a regular table which I later delete. This would mean a lot of writing to disks but it's something that the system I work with would survive, but not be happy with. Is there any other way to handle this?

Answer 1

A temporary table only persists for the duration of the scope that declared it. For a "normal" connection that will be when the connection is dropped. For example, if you're using SSMS and open a query window and run CREATE TABLE #T (ID int); it'll create the table. As you're still connected, the table won't be dropped and will still exist. If you run the statement again (without dropping it) you'll get an error that it already exists. As soon as you close that query window, the temporary table will be dropped.
For a dynamic statement, the scope is the duration of that dynamic statement. This means that as soon as the dynamic statement completes, the table will be dropped:
EXEC sys.sp_executesql N'CREATE TABLE #T (ID int);';
SELECT *
FROM #t;
Notice this errors, as the scope the table was created in has completed, and thus dropped.
If you are using dynamic statements to create temporary tables, you need to make all the references to said temporary table within the dynamic statement.
Otherwise, if you need to reference it outside of the statement, I personally find I create an "permanent" object in tempdb, and then clean up afterwards.
EXEC sys.sp_executesql N'CREATE TABLE tempdb.dbo.T (ID int);';
SELECT *
FROM tempdb.dbo.T;
DROP TABLE tempdb.dbo.T;
These tables are still dropped in the event the instance is restarted as well.
Note that "global" temporary table behave slightly differently. As global temporary table can be referenced in any connection, while it exists. This means that another connection could be using the table while the scope that created it ends. As a result a global temporary table persists until the scope that declared is ends and there are no other active connections using the object. This means that the objects could be dropped mid batch in another connection.