Why Are My nvarchar(max) Strings Truncated in SQL Server?

Truncation of nvarchar(max) Strings

When attempting to concatenate large strings in SQL Server using the nvarchar(max) data type, users may encounter unexpected truncation to 4000 characters. This issue stems from implicit conversion.

Implicit Conversion Issue

When concatenating Unicode/nChar/nVarChar values, SQL Server implicitly converts the resulting string to nVarChar(4000). This implicit conversion occurs even if the target variable is defined as nvarchar(max). Unfortunately, SQL Server fails to provide a warning or error when truncation occurs, potentially leading to incorrect data.

Solution: Force Explicit Conversion

To prevent this issue, it is crucial to force explicit conversion to nVarChar(MAX) when building the string. This can be achieved by prefacing the string with CAST('' as nVarChar(MAX)) as follows:

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.

Why the Implicit Conversion Issue Occurs

Behind the scenes, SQL Server evaluates the right-hand side of the assignment first, resulting in implicit conversion to nVarChar(4000). After this conversion, the string is assigned to the nvarchar(max) variable, but by that point, truncation has already occurred.

Note on Literal Strings

It is important to note that literal strings (i.e., hard-coded strings enclosed in apostrophes) have a maximum length of 4000 characters. Breaking such strings up into smaller segments may be necessary to avoid truncation.

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