What is the difference between varchar and nvarchar?

VARCHAR vs. NVARCHAR: Understanding String Data Types in SQL Server

Explore the fundamental differences between VARCHAR and NVARCHAR data types in SQL Server, focusing on storage, character sets, and performance implications for effective database design.

When designing a database in SQL Server, choosing the correct data type for string storage is crucial. Two of the most commonly used string data types are VARCHAR and NVARCHAR. While they both store variable-length character data, their underlying storage mechanisms, character set support, and performance characteristics differ significantly. Understanding these distinctions is key to optimizing your database for internationalization, storage efficiency, and query performance.

VARCHAR: ASCII and Single-Byte Character Storage

VARCHAR (Variable-length character string) is designed to store non-Unicode character data. This means it typically uses a single-byte encoding (like ASCII or a specific code page) where each character occupies 1 byte of storage. The maximum length for VARCHAR is 8,000 characters. If you need to store more, you can use VARCHAR(MAX), which can store up to 2 GB of data, similar to a TEXT data type.

CREATE TABLE Products (
    ProductID INT PRIMARY KEY,
    ProductName VARCHAR(255) -- Stores up to 255 non-Unicode characters
);

INSERT INTO Products (ProductID, ProductName)
VALUES (1, 'Laptop'); -- 'Laptop' uses 6 bytes of storage

Example of VARCHAR usage in a SQL Server table.

NVARCHAR: Unicode and Multi-Byte Character Storage

NVARCHAR (National character varying) is designed to store Unicode character data. This is its primary distinction from VARCHAR. Unicode is a universal character encoding standard that supports characters from virtually all writing systems of the world. To achieve this, NVARCHAR typically uses 2 bytes per character (UTF-16 encoding in SQL Server). The maximum length for NVARCHAR is 4,000 characters. Similar to VARCHAR(MAX), NVARCHAR(MAX) can store up to 2 GB of data.

CREATE TABLE InternationalProducts (
    ProductID INT PRIMARY KEY,
    ProductName NVARCHAR(255) -- Stores up to 255 Unicode characters
);

INSERT INTO InternationalProducts (ProductID, ProductName)
VALUES (1, N'Laptop'), (2, N'ノートパソコン'); -- 'ノートパソコン' (Japanese) requires NVARCHAR

Example of NVARCHAR usage, including Unicode characters.

Key Differences and Performance Implications

The choice between VARCHAR and NVARCHAR boils down to character set support and storage efficiency. Here's a summary of their core differences and how they impact performance:

flowchart TD
    A[Data Type Choice] --> B{Character Set Required?}
    B -->|Non-Unicode (e.g., English)| C[VARCHAR]
    B -->|Unicode (e.g., Multi-language)| D[NVARCHAR]

    C --> C1[Storage: 1 byte/char]
    C --> C2[Max Length: 8000 chars]
    C --> C3[Performance: Potentially faster for single-byte data]

    D --> D1[Storage: 2 bytes/char]
    D --> D2[Max Length: 4000 chars]
    D --> D3[Performance: Slower due to increased storage/processing for same char count]

Decision flow for choosing between VARCHAR and NVARCHAR.

Storage and Performance

• Storage: NVARCHAR uses twice the storage space of VARCHAR for the same number of characters because it stores each character using 2 bytes. This directly impacts disk space, I/O operations, and memory usage.
• Indexing: Larger data types mean larger indexes. An index on an NVARCHAR column will be roughly twice the size of an index on a VARCHAR column with the same data, potentially leading to more disk I/O during index scans and seeks.
• Query Performance: While modern SQL Server versions are highly optimized, operations like string comparisons, sorting, and pattern matching on NVARCHAR columns can be slightly slower due to the increased data size and the complexity of Unicode collation rules. However, for most applications, the performance difference might be negligible unless dealing with extremely large datasets or highly intensive string manipulations.

Collation

Collation defines the rules for how character data is sorted and compared. VARCHAR columns use the database's default collation (or a specified collation) which is typically non-Unicode. NVARCHAR columns always use Unicode collations, which are designed to handle the complexities of various language-specific sorting and comparison rules.