Dynamic SQL Not Converting VARCHAR To INT (shouldn't anyway)

Question

I'm receiving an error:

Conversion failed when converting the varchar value 'INSERT INTO TableRowCount (IntFieldID, DecimalField) SELECT 'to data type int"

Using the following code:

DECLARE @start INT -- @start is an INT
SET @start = 1 -- INT

DECLARE @sql NVARCHAR(MAX)
SET @sql = 'INSERT INTO TableRowCount (IntFieldID, DecimalField)
SELECT ' + @start +', COUNT(*)
FROM dbo.somewhere' -- location is irrelevant

EXECUTE(@sql) -- this is where it fails

If I remove IntFieldID and the @start, it will work with an insert (though it defeats the purpose). I've tried including a SELECT CAST(' + @start + ' AS INT), which seems a little redundant since @start is an INT already (casting an INT as an INT), but that doesn't work either. I also tried beginning with an N' DYNAMIC-SQL, which didn't work, I tried using three ''' around everything (didnt' work), and in a few places that I read online, responses suggested putting the variable in the string, which generated the error:

Must declare scalar variable @start

(no surprise, as that didn't sound correct).

Solution 1

You need to convert your @Start to a varchar.

DECLARE @sql NVARCHAR(MAX)
SET @sql = 'INSERT INTO TableRowCount (IntFieldID, DecimalField)
SELECT ' + CAST(@start as nvarchar(20)) +', COUNT(*)
FROM ' + @conn

SQL Server implicitly converts between datatypes on concatenation or addition based on some fairly complex criteria. Suffice to say if you try to combine an int and a string it will always attempt to convert the string to an int unless you tell it otherwise explicitly.

Below is a conversion chart for your reference from MSDN.

Solution 2

A better way than trying to concatenate an integer is to pass it in as a strongly-typed parameter:

DECLARE @start INT = 1;

DECLARE @sql NVARCHAR(MAX) = N'INSERT ...
  SELECT @start, COUNT(*) FROM ' + @conn;

EXEC sp_executesql @sql, N'@start INT', @start;