Why Does NVARCHAR(MAX) Still Truncate My Strings in SQL Server?

"NVARCHAR(MAX) Truncation Still Occurs: Understanding Implicit Conversion"

Despite the misconception that NVARCHAR(MAX) in modern SQL Server versions holds substantial data, users may encounter truncation to 4000 characters like in this instance:

DECLARE @Query NVARCHAR(max);
SET @Query = 'SELECT...' -- some of the query gets set here
SET @Query = @Query + '...' -- more query gets added on, etc.

-- later on...
PRINT LEN(@Query) -- Prints out 4273, which is correct as far as I can tell
PRINT @Query      -- Truncates value to 4000 characters
EXEC sp_executesql @Query -- totally crashes due to malformed (truncated) query

The Root Cause: Implicit Conversion

The issue stems from implicit conversion. When concatenating strings with Unicode characters, SQL Server automatically converts the result to NVARCHAR(4000) without warning or indication of truncation. This occurs even when the variable declared to store the result is NVARCHAR(MAX).

Solution: Enforce Explicit Conversion

To prevent implicit conversion and ensure the retention of large data, always pre-concatenate using CAST:

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.

By casting an empty string to NVARCHAR(MAX), SQL Server is explicitly instructed to handle the entire string as NVARCHAR(MAX). This eliminates the truncation issue.

Other Considerations

• Avoid literal strings longer than 4000 characters (or 8000 for VarChar) to prevent truncation.
• Break up large strings into smaller chunks to ensure successful concatenation.

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