TSQL SET command not working inside EXEC statement - sql

Question

I am trying to run the below script in SQL Server Management Studio:
DECLARE #counter INT = 1
EXEC('SET '+#counter+' = '+#counter+' + 1')
However, I get this error:
Msg 102, Level 15, State 1, Line 1
Incorrect syntax near '1'-
I have been troubleshooting for days but gotten nowhere - I guess most likely TSQL simply does not accept this inside an EXEC command, but what could I potentially replace it with?
This is a part of a larger script where the above MUST be inside an EXEC statement in order for the rest to work..

Answer 1

This isn't going to working for a number of reasons.
Let's start with what you have:
DECLARE #counter INT = 1
EXEC('SET '+#counter+' = '+#counter+' + 1')
EXEC is expecting a string inside the parenthesis, yet you have an int (the variable #counter), so that is your first problem. Oddly, however, you aren't getting a conversion error here; it seems that #counter is being interpreted as its value (1), rather than Data Type Precedence being applied first. (If you try SELECT 'SET '+#counter+' = '+#counter+' + 1' you'll get the error "Conversion failed when converting the varchar value 'SET ' to data type int.").
Even if the query successfully got past that hurdle, that would cause another error because the query becomes:
SET 1 = 1 + 1
Obviously that isn't going to work, 1 is 1, no other number.
Next, a variable declared outside of the scope of a "dynamic" statement can't be referenced inside it. Let's take the following:
DECLARE #Varible int = 1;
EXEC (N'SELECT #Variable;');
This returns the error:
Must declare the scalar variable "#Variable".
If you are using a "dynamic" statement (your statement isn't dynamic at all), then you need to parametrise the variables. This is why you shouldn't be using EXEC ({SQL Statement}) and should be using sys.sp_executesql.
So, for the above, you get:
DECLARE #counter INT = 1;
DECLARE #SQL nvarchar(MAX);
SET #SQL = N'SET #Variable = #Variable + 1;';
EXEC sys.sp_executesql #SQL, N'#Variable int OUTPUT',#Variable = #counter OUTPUT;
SELECT #Counter;
As I mentioned, that is all pointless, as there's nothing dynamic in the above, and your query should just be:
DECLARE #counter INT = 1;
SET #counter = #counter + 1;
Of course, none of this even goes on to address things like how to safely inject objects into a dynamic statement, but that isn't what this question is about. As I mention, the dynamic statement here is redundant, as it isn't dynamic, and it's not that required deferred parsing (such as when you ALTER a table to add a column, and then try to reference that column in the same batch).

Answer 2

If you run the script below, it'll throw an error like
Msg 137, Level 15, State 1, Line 1
Must declare the scalar variable "#counter".
BEGIN
DECLARE #counter INT = 1
EXEC('SET #counter = '+ #counter+' + 1')
END
WHY?
Because dynamic SQL is a different scope to the outer.
So can try this way (using output)
DECLARE #counter INT = 1
DECLARE #SQL nvarchar(50) = 'SELECT #counter2 =' + CAST(#counter + 1 AS VARCHAR(100))
exec sp_executesql #SQL, N'#counter2 int out', #counter out
SELECT #counter