Why is my nvarchar(max) variable truncated to 4000 characters when printed in SQL Server 2008?

nvarchar(max) Truncation: Unraveling the Implicit Conversion Trap

In the realm of SQL Server 2008 stored procedures, we often encounter the task of dynamically generating long, complex queries. To accommodate such scenarios, we rely on variables of the renowned nvarchar(max) datatype, which is touted to possess the ability to store vast amounts of data. However, a common misconception arises when attempting to retrieve the value of such variables.

Upon printing the value of an nvarchar(max) variable, you might find it mysteriously truncated to a mere 4000 characters. This puzzling behavior stems from a hidden pitfall: implicit conversion.

When concatenating Unicode or nChar/nVarChar values, SQL Server secretly converts the resulting string to nVarChar(4000), even when your variable is of the nvarchar(max) datatype. This conversion, unbeknownst to us, leads to the premature truncation of our query.

To circumvent this implicit conversion trap, it is imperative to explicitly force the concatenation to nvarchar(max) before any operations take place. This can be achieved by prefacing the concatenation with CAST('' as nVarChar(MAX)). By casting the empty string to nVarChar(MAX) and concatenating it to your query, you instruct SQL Server to maintain the larger datatype throughout your query building process.

Consider the following code snippet:

SET @Query = CAST('' as nVarChar(MAX)) -- Force implicit conversion to nVarChar(MAX)
+ 'SELECT...'
+ '...'
PRINT LEN(@Query)
PRINT @Query

Now, when you print the value of @Query, it will accurately reflect its full length, preventing any unexpected truncation. This technique ensures that your queries remain intact, allowing for seamless execution and accurate results.

Therefore, remember to pre-concatenate your nvarchar(max) variable with CAST('' as nVarChar(MAX)) whenever building long, dynamic queries. This simple yet crucial step will save you from the treacherous pitfalls of implicit conversion, preventing data truncation and safeguarding your SQL Server code.

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