How to run a more than 8000 characters SQL statement from a variable?

Question

I can use the following code for tiny little queries:

DECLARE @sql VARCHAR(8000)
SET @sql = 'SELECT * FROM myTable'
Exec @sql

The above method is very useful in order to maintain large amounts of code, especially when we need to make changes once and have them reflected everywhere.

My problem is my query (it's only one single query) that I want to feed into the @sql variable uses more than 25 table joins, some of them on temporary table variables, incorporates complex operations and it is hence much more than 8000 characters long.

I wished to use TEXT data type to store this query, but MSDN shows a warning message that Microsoft is planning to remove Text, NText and Image data types from their next versions. I wish my code to run in future too.

I thought of storing this query in a separate file, but as it uses joins on table variables and other procedure-specific parameters, I doubt if this is possible.

Kindly tell me a method to store a large query into a variable and execute it multiple times in a procedure.

Answer 1

The problem is with implicit conversion.

If you have Unicode/nChar/nVarChar values you are concatenating, then SQL Server will implicitly convert your string to VarChar(8000), and it is unfortunately too dumb to realize it will truncate your string or even give you a Warning that data has been truncated for that matter!

When concatenating long strings (or strings that you feel could be long) always pre-concatenate your string building with CAST('' as nVarChar(MAX)) like so:

SET @Query = CAST('' as nVarChar(MAX))--Force implicit conversion to nVarChar(MAX)
           + 'SELECT...'-- some of the query gets set here
           + '...'-- more query gets added on, etc.

What a pain and scary to think this is just how SQL Server works. :(

I know other workarounds on the web say to break up your code into multiple SET/SELECT assignments using multiple variables, but this is unnecessary given the solution above.

For those who hit a 4000 character max, it was probably because you had Unicode so it was implicitly converted to nVarChar(4000).

Explanation:
What's happening behind the scenes is that even though the variable you are assigning to uses (MAX), SQL Server will evaluate the right-hand side of the value you are assigning first and default to nVarChar(4000) or VarChar(8000) (depending on what you're concatenating). After it is done figuring out the value (and after truncating it for you) it then converts it to (MAX) when assigning it to your variable, but by then it is too late.