Question

I want to know if its possible to set a declare variable (as table), to null.

like :

DECLARE @TABLE1 AS TABLE (COL1 INT)
SET @TABLE1 = NULL

I searche a way to know if I already try to insert data into my variable

Answer 1

To complete your question about if you’ve added rows or not you can do:

insert into @TABLE1 (COL1) VALUES ([someYourData])

if @@ROWCOUNT > 0
   -- row addeded
ELSE
   -- nothing added

or you can check the value of @@ROWCOUNT to know how many rows has been added so:

insert into @TABLE1 (COL1) VALUES ([someYourData])
print @@ROWCOUNT

For what i know @TABLE1 have a local scope so when the Store Procedure is end, this variable will be deleted

and as GSerg said you have many whays to check it as:

if exists (select * from @TABLE1)
BEGIN
   --  do something
END

or

if (select count(@@rowcount) from @TABLE1) > 0
BEGIN
   --  do something
END

or

if (select count(*) from @TABLE1) > 0
BEGIN
   --  do something
END