Concatenating Strings, NULLs and EXEC – measure twice, cut once

G’day,

SQL Server’s great – its been teaching me a lot for many years. But every so often you get a surprise. Sometimes it’s a nice one, sometimes it more of the ‘really, are you sure kind’ – and then you realise, yes SQL Server – you’re correct, I really should have known that.

This one involved

A cursor
String concatenation
Executing a T-SQL Statement

The first two of these didn’t suprise me – after I’d thought about it.

The last one did

I’ll attempt to give an example of what occured

I have a table

USE tempdb;
GO

IF OBJECT_ID('dbo.Issue') IS NOT NULL
	BEGIN
		DROP TABLE dbo.Issue
	END;


CREATE TABLE dbo.Issue
(
	SomeName NVARCHAR(50) NULL
);

and, we’re inserting into that table using a concatenated T-SQL string, like so

DECLARE @InsertValue NVARCHAR(50) = N'TestValue';
DECLARE @SQL NVARCHAR(500) = 'INSERT INTO dbo.Issue(SomeName) VALUES(''' + @InsertValue + ''');';
PRINT @SQL;
EXEC(@SQL);

As you’d expect one row goes in

However, the issue that I saw arose when the variable being INSERTed was NULL, like so

DECLARE @InsertValue NVARCHAR(50) = NULL;
DECLARE @SQL NVARCHAR(500) = 'INSERT INTO dbo.Issue(SomeName) VALUES(''' + @InsertValue + ''');';
PRINT @SQL;
EXEC(@SQL);

You’ll notice (if you run the code) that nothing gets printed – which is fair enough as a NULL in a string concatenation will unltimatly yeild a NULL, as anything + an unknown (NULL) is an unknown (NULL)

That wasn’t the surprising part – although it did get me to think a bit at first.

The surprising part (to me at least), was that the string – which is now NULL -has been run inside the EXEC(..) call, had not errored, had not failed, but had not inserted anything either

Try it

DECLARE @InsertValue NVARCHAR(50) = NULL;
DECLARE @SQL NVARCHAR(500) = 'INSERT INTO dbo.Issue(SomeName) VALUES(''' + @InsertValue + ''');';
PRINT @SQL;
EXEC(@SQL);
SELECT * FROM dbo.Issue;

You can try this more simply by executing a NULL string

DECLARE @Test NVARCHAR(20) = NULL;
EXEC(@Test);

Note however, that you can’t directly execute a NULL (although why you’d want to is a different matter 🙂 )

EXEC(NULL)

Msg 156, Level 15, State 1, Line 44
Incorrect syntax near the keyword 'NULL'.

So, now you know.

If you’ve never seen this before (or even if you have, but a long time ago) then this could easily make you wonder what is going on.

I hope this helps somebody.

Have a great day.

Cheers

Martin.

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