A data type is a particular kind of data item that is defined by the values it can take and the operations that can be performed on it. SQL supports various data types, including numeric, date and time, character and string, binary, and more. The choice of data type affects data integrity, storage, and performance; the choice of the optimal data type is not always all that obvious.
This article will focus on three commonly used data types: CHAR, VARCHAR, and VARCHAR(MAX), comparing and discussing their characteristics and uses.
- Character data types in SQL
- CHAR – fixed-length data type
- VARCHAR – variable-length data type
- VARCHAR(MAX) – maximum text storage
- TEXT – deprecated large text data type
- Differences between CHAR, VARCHAR, and VARCHAR(MAX)
- Common errors in working with character data types
- How to choose the right data type
- Conclusion
Character data types in SQL
Character data types store alphanumeric data, including letters, numbers, symbols, and whitespaces. These types can be either fixed-size (CHAR) or variable-size (VARCHAR). Since SQL Server 2019, character data types support the full range of Unicode characters using the UTF-8 encoding. However, if a non-UTF-8 collation is applied, CHAR and VARCHAR will only store the subset of characters supported by the corresponding code page of that collation.
The choice of the optimal character data types matters for several reasons:
- Query performance: The choice of data type can impact performance. For instance, CHAR can be faster because it avoids length calculations, whereas VARCHAR is preferable for columns with varying data lengths.
- Data integrity: Character data types ensure that text data meets specific rules, like adhering to a maximum length. However, this can sometimes cause compatibility issues with other databases or systems.
- Storage efficiency: The data type directly affects storage requirements. For example, CHAR(50) always allocates 50 bytes, regardless of the actual data length, while VARCHAR(50) uses only the space needed for the actual data plus 2 bytes.
Let’s dive deeper into the specifics of character data types.
CHAR – fixed-length data type
The CHAR(n) data type is designed for fixed-length, non-Unicode character data, where n specifies the string size in bytes (ranging from 1 to 8,000). It stores any character—letters, numbers, symbols, and even the null character. If the stored data is shorter than the defined length, the database pads it with spaces to meet the fixed length.
CHAR data types work well for data that maintains a consistent length, such as phone numbers or postal codes.
Advantages of CHAR
- Uniform data length: CHAR is good for storing standardized identifiers of a consistent size
- Predictable storage: Fixed length makes it easier to determine the required storage and optimize performance
- Faster access: There is no need for variable-length calculations, which offers slight speed gains
- Simpler indexing: Fixed-length data allows for uniform index entries, which can improve indexing speed
- Reduced row fragmentation: In high-update environments, CHAR columns are less prone to fragmentation
- Data integrity: CHAR can store spaces instead of NULLs, enabling non-null constraints without complex handling for empty values
Disadvantages of CHAR
- Padding with spaces: Data shorter than the defined length is automatically padded with spaces, potentially complicating retrieval
- Higher disk usage: CHAR columns can consume more storage, especially when many entries are shorter than the defined length
- Reduced flexibility: Operations like pattern matching may require extra trimming due to trailing spaces
- Fixed design: Changing the length of a CHAR column requires a schema alteration, which can be cumbersome
CHAR is commonly used in small, frequently queried lookup tables, such as those for status tracking or code validation. Fixed length improves performance, making CHAR ideal when data integrity and predictable schemas are more critical than storage efficiency. However, CHAR is less suitable for data of varying lengths, where it is recommended to use VARCHAR as a more flexible and storage-efficient option.
VARCHAR – variable-length data type
VARCHAR is a variable-length string data type with a maximum length of 8,000 characters. It can store any characters, including numbers, letters, special characters, non-printing characters, and the ASCII null character.
Each character in a VARCHAR string uses 1 byte. Unlike fixed-length columns, VARCHAR only occupies the space needed for the actual data stored, without padding. Although it may perform slightly slower than CHAR due to length calculations, VARCHAR provides significant storage savings.
Advantages of VARCHAR
- Efficient storage: VARCHAR uses space only for the data stored, unlike CHAR, which always allocates a fixed size, even when data is shorter
- Flexibility: VARCHAR is great for variable-length data like names, addresses, and descriptions
- Improved performance for smaller data: With no need to process empty spaces, calculations avoid unnecessary I/O and memory use
- Adjustable length: You can set a maximum length (like VARCHAR(255)), allowing control over storage based on expected data size
Disadvantages of VARCHAR
- Potential performance impact: Handling variable lengths can increase memory management complexity and slow down data retrieval from large tables
- Indexing challenges: Indexes on VARCHAR columns may be slower and less efficient than those on fixed-length columns, especially in large datasets
- Sorting and comparison overhead: Sorting and comparing VARCHAR data can require extra processing due to variable lengths
- Inefficiency for consistently short data: If data in a VARCHAR field is usually shorter than the maximum length, CHAR may be more efficient
- Risk of data truncation: If the data length exceeds the specified maximum, truncation can occur, leading to data loss
While VARCHAR offers numerous benefits, it may not be the best choice for large-scale applications with heavy indexing or for data with minimal length variation. In such cases, CHAR or another data type might be more efficient.
VARCHAR(MAX) – maximum text storage
The VARCHAR(MAX) data type supports variable-length character strings up to 2 GB, making it ideal for storing large texts.
Key points to note:
- VARCHAR(MAX) columns do not allow a fixed length limit
- VARCHAR(MAX) columns cannot be used as key columns in an index
Advantages of VARCHAR(MAX)
- Flexible data size: With a capacity of up to 2 GB, VARCHAR(MAX) is suitable for applications needing to store highly variable text, like comments or notes
- Optimized space: Small VARCHAR(MAX) values are stored in-row with other data and are moved off-row when data exceeds 8 KB, storing a pointer in place
- Reduced schema changes: Using VARCHAR(MAX) minimizes the need for schema updates as data grows
- Storing JSON or XML data: VARCHAR(MAX) is well-suited for storing large, variable-length JSON or XML data
Disadvantages of VARCHAR(MAX)
- Potential performance impact: VARCHAR(MAX) can slow down query performance, especially when large data storage is unnecessary
- Limited indexing: VARCHAR(MAX) columns cannot be directly indexed, which restricts their effectiveness in search, filter, and sort operations
- High memory usage: Loading multiple large strings can strain server memory while moving data off-row requires extra storage for pointers
- Compatibility concerns: Operations like GROUP BY and DISTINCT may not be fully optimized for VARCHAR(MAX) columns with highly variable lengths
- Locking conflicts: Storing and updating large data in VARCHAR(MAX) columns can cause table or page locks, degrading performance
- Risk of data overload: Without a length limit, it’s easy to insert more data than necessary, risking data consistency
The VARCHAR(MAX) data type helps handle large text data but should be used carefully. It’s most suitable when data lengths vary widely and can exceed 8,000 bytes. For smaller, predictable text sizes, fixed-length CHAR fields are often more efficient.
TEXT – deprecated large text data type
The TEXT data type is used to store large amounts of text data, including both single-byte and multibyte characters supported by the locale. A table can include up to 195 columns of the TEXT data type.
In SQL Server, the TEXT data type has been deprecated since SQL Server 2005. Microsoft recommends using VARCHAR(MAX) or NVARCHAR(MAX) for development, as support for the TEXT data type will be removed in the upcoming version of SQL Server.
Differences between CHAR vs VARCHAR vs VARCHAR(MAX)
The below table describes the differences between the CHAR, VARCHAR, and VARCHAR(MAX) data types:
| Feature | CHAR | VARCHAR | VARCHAR(MAX) |
|---|---|---|---|
| Data type | Fixed-length string data | Variable-length string data | Variable-length string data with large capacity |
| Storage allocation | Allocates defined space regardless of data | Allocates space based on content (actual data size + 2 bytes) | Allocates space based on content (actual data size + 2 bytes) |
| Maximum length | Up to 8,000 bytes | Up to 8,000 bytes | Up to 2 GB |
| Padding | Padding with spaces up to the defined length | No padding | No padding |
| Performance | Faster for fixed-length data due to less overhead | Slightly slower due to variable length handling | Slower due to potentially large size and using LOB (Large Object) storage |
| Use case | Short, fixed-length data (codes, flags, etc.) | Variable-length, constrained data (names, addresses, etc.) | Large text data, beyond 8,000 characters (comments, posts, etc.) |
Common errors in working with character data types
Working with CHAR, VARCHAR, and VARCHAR(MAX) can lead to specific errors that database specialists should keep in mind. Below, we’ll cover the most common issues associated with these data types and how to address them.
- Truncation errors: Truncation occurs when a user attempts to insert a string longer than the defined length of a CHAR or VARCHAR column, resulting in data being cut off. To prevent this, ensure that the length of the data matches the allowed length for the column.
- Misuse of VARCHAR(MAX): VARCHAR(MAX) is often misused, such as for columns containing small, fixed-size data, where CHAR is appropriate. It’s crucial to match the data type with the intended data.
- Trailing spaces in CHAR: The CHAR data type pads strings with spaces to reach the specified length, which can lead to unexpected behavior in comparisons and JOINs. To avoid these issues, use VARCHAR for variable-length strings where padding is unnecessary.
- Data migration and compatibility issues: Migrating data from other systems may introduce inconsistencies in CHAR and VARCHAR values, leading to truncation or padding problems. Standardize data lengths and formats before beginning the migration process to minimize these issues.
How to choose the right data type
Previously, we discussed various aspects of the CHAR vs VARCHAR vs VARCHAR(MAX) usage. Choosing the right type depends on data characteristics, length requirements, storage, and performance considerations. The following table summarizes this information.
| CHAR | VARCHAR | VARCHAR(MAX) | |
|---|---|---|---|
| When to use | When your data always has a consistent length | When data lengths vary but have a defined maximum length under 8000 characters | When data length is highly variable and can exceed 8000 characters |
| Typical use cases | ISO country codes, abbreviations, product codes, reference numbers, serial numbers, etc. | Names, addresses, email addresses, descriptions, comments, product names, etc. | Detailed descriptions, documents, logs, articles, lengthy comments, JSON and XML data |
| Specifics | Best for large datasets with data of small fixed length | Allocating only the required space for the actual data size | Suitable if data is expected to grow in length over time |
| Advantages | Predictable storage requirements and faster performance for larger datasets | More efficient storage for the data of variable length and reducing the wasted space | Allows storing up to 2GB of data without setting any hard limits |
| Considerations | Padding the strings with spaces to match the defined length may waste space | The maximum length is 8000 characters; if the data exceeds it, refer to VARCHAR(MAX) | Possible performance issues due to the size of data stored as Large Objects (LOBs) |
Conclusion
Considering CHAR vs VARCHAR vs VARCHAR(MAX) when managing database tables is essential. This article covered these data types along with their unique characteristics to help you choose the right option for each particular case.
Besides, database-related tasks all become easier with the right tools, such as dbForge Studio for SQL Server. It is a powerful all-in-one solution for SQL Server professionals that offers an intuitive visual Data Editor, which helps manage data of all types in the most convenient way as well as perform all other database development, management, and administration tasks.
The fully functional trial of the Studio is available for 30 days, so feel free to download it and utilize all the robust capacities for your daily tasks.
