What are the limits and truncation behaviors of SQL NVARCHAR and VARCHAR, and how can I resolve truncation issues in dynamic SQL queries?

Understanding SQL NVARCHAR and VARCHAR Limitations and Truncation

Dynamic SQL queries often hit unexpected string length limitations. While NVARCHAR(MAX) theoretically holds over 2GB of data, practical limitations exist, especially with concatenation. This article clarifies these limits and provides solutions for preventing truncation.

Truncation Behavior in Concatenation

The outcome of string concatenation depends heavily on the data types involved:

• varchar(n) varchar(n): Truncation occurs at 8,000 characters.
• nvarchar(n) nvarchar(n): Truncation occurs at 4,000 characters.
• varchar(n) nvarchar(n): Truncation occurs at 4,000 characters.
• [n]varchar(max) [n]varchar(max): No truncation (below 2GB limit).
• varchar(max) varchar(n): No truncation (below 2GB limit), resulting in varchar(max).
• varchar(max) nvarchar(n): varchar(n) is converted to nvarchar(n). Truncation to nvarchar(4000) occurs if varchar(n) exceeds 4,000 characters.
• nvarchar(max) varchar(n): varchar(n) is converted to nvarchar(n). No truncation (below 2GB limit).

Data Types of String Literals

• N'string' (with N prefix): Treated as nvarchar(n), where n is the string length.
• 'string' (without N prefix):
  • varchar(n) if length ≤ 8,000 characters.
  • varchar(max) if length > 8,000 characters.

Preventing Truncation

To avoid truncation issues:

• Use NVARCHAR(MAX) consistently: Concatenate only NVARCHAR(MAX) strings.
• Prefix literals: For strings between 4,001 and 8,000 characters, use the N prefix.
• Initialize with NVARCHAR(MAX): Declare variables as NVARCHAR(MAX) from the start:

<code class="language-sql">DECLARE @SQL NVARCHAR(MAX) = '';
SET @SQL = @SQL + N'Foo' + N'Bar';</code>

Inspecting Truncated Strings

To fully view potentially truncated strings:

• Switch to "Results to Grid" view.
• Employ this query:

<code class="language-sql">DECLARE @SQL NVARCHAR(MAX) = '';
SET @SQL = @SQL + N'Foo' + N'Bar';</code>

This approach effectively bypasses length restrictions for display purposes.

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