In SQL Server, a computed data type is a virtual column in a table that is not physically stored but is dynamically calculated based on the values of other columns in the same table. Computed columns are useful when you need to perform calculations on existing data and store the result for future use.
Creating Computed Columns
To create a computed column, you need to specify an expression that defines how the column’s value should be calculated. This expression can be based on one or more columns in the same table.
CREATE TABLE TableName ( Column1 DataType, Column2 DataType, ComputedColumn AS (Expression) )
Let’s consider an example where we have a table called “Employees” with columns “FirstName”, “LastName”, and a computed column “FullName” that concatenates the first and last names:
CREATE TABLE Employees ( FirstName VARCHAR(50), LastName VARCHAR(50), FullName AS (FirstName + ' ' + LastName) )
Benefits of Computed Columns
- Data Integrity: Computed columns ensure that derived values are always up-to-date, reducing the risk of inconsistent or incorrect data.
- Performance Optimization: Since computed columns are calculated at runtime, they can help optimize query performance by pre-calculating frequently used values.
- Data Consistency: By using computed columns, you can enforce consistent data across multiple tables by basing their values on related tables’ columns.
Although computed columns provide convenience and performance benefits, it’s important to note that they have limitations:
- Non-Deterministic Functions: Computed columns cannot depend on non-deterministic functions like GETDATE() or NEWID().
- Aggregate Functions: Computed columns cannot use aggregate functions like SUM() or COUNT().
- UDFs: If the computed column expression includes a user-defined function (UDF), the UDF must be deterministic.
In SQL Server, computed columns are a powerful feature that allows you to derive values based on other columns in a table. They offer data integrity, performance optimization, and data consistency benefits. However, it’s important to consider their limitations when using them in your database design.
By understanding how to create and utilize computed columns effectively, you can enhance your SQL Server database’s functionality and improve overall query performance.