SQL Server Documentation

MSDN Documentation > SQL Server > Data Types > sql_variant

sql_variant Data Type

sql_variant is a data type that can store values of various SQL Server–supported data types, except for the following: text, ntext, image, xml, timestamp, sql_variant itself, table, geography, geometry, hierarchyid, uniqueidentifier, or CLR user-defined types. It allows for flexibility in designing tables where columns need to store different types of data.

Overview

The sql_variant data type is useful when you want to create a single column that can store different data types. For example, you might use it in a table that tracks various kinds of user-defined properties where the data type of the property value can differ from one property to another. When a value is inserted into a sql_variant column, the data type of the value is stored along with the value itself.

When a sql_variant value is retrieved, it can be implicitly or explicitly converted to its original data type.

Syntax

There is no specific syntax for creating a sql_variant column. You declare it like any other data type: 

CREATE TABLE ExampleTable (
    ID INT PRIMARY KEY,
    PropertyData sql_variant
);

When inserting data, the system automatically handles the type storage: 

INSERT INTO ExampleTable (ID, PropertyData)
VALUES (1, 'Some Text');

INSERT INTO ExampleTable (ID, PropertyData)
VALUES (2, 12345);

INSERT INTO ExampleTable (ID, PropertyData)
VALUES (3, GETDATE());

Key Characteristics and Usage

  • Type Safety: While flexible, sql_variant maintains type information. This allows for type-safe comparisons and conversions.
  • Storage Overhead: Storing data as sql_variant incurs some storage overhead because the original data type information must also be stored.
  • Performance Considerations: Operations on sql_variant columns can sometimes be slower than operations on columns with fixed, specific data types due to the overhead of type checking and conversion.
  • Indexing: You can create indexes on sql_variant columns, but care must be taken as the index's performance can be affected by the variety of data types stored.

Functions for sql_variant

SQL Server provides several functions to work with sql_variant data:

  • CAST and CONVERT: Used to convert a sql_variant value to its original data type or another compatible type.
  • SQL_VARIANT_PROPERTY(): Returns the property of a sql_variant value. The common properties include base_type (the original data type), precision, and scale.
  • ISDATE(): Checks if a sql_variant value can be converted to a date.
  • ISNUMERIC(): Checks if a sql_variant value can be converted to a numeric type.

Example using SQL_VARIANT_PROPERTY()

SELECT
    ID,
    PropertyData,
    SQL_VARIANT_PROPERTY(PropertyData, 'base_type') AS DataType
FROM
    ExampleTable;

Considerations and Best Practices

Note: Use sql_variant judiciously. If you know the data type of a column in advance, it's generally better to use that specific data type for better performance and data integrity.
Tip: When querying sql_variant data, it's often beneficial to use CASE statements or IF conditions to handle different data types explicitly, or use SQL_VARIANT_PROPERTY() to determine the type before performing operations.
  • Performance Tuning: If sql_variant columns are frequently queried, consider creating appropriate indexes and testing performance.
  • Data Validation: Implement robust validation logic to ensure that only expected data types are inserted into the sql_variant column.