Computed columns in SQL Server

 Computed columns in SQL Server are columns in a table that are defined by an expression or formula rather than storing data directly. These columns are automatically calculated based on other columns in the same table, allowing you to avoid redundant data storage and ensure data consistency.

Key Characteristics of Computed Columns

1. Definition: Computed columns are defined using an expression that calculates their values based on other columns in the same table.

2. Persistence: Computed columns can be either persisted or non-persisted:

   • Persisted: The computed column's value is physically stored in the database, which can improve query performance but requires additional storage space. Changes to the dependent columns cause the computed column to be automatically updated.
   • Non-persisted: The computed column's value is calculated on the fly when queried, saving storage space but potentially impacting query performance since the calculation is performed each time the column is accessed.

3. Data Types: Computed columns can use any valid expression and can be of various data types, depending on the result of the expression.

4. Indexes: You can create indexes on computed columns, which can improve performance for queries that use these columns.

5. Constraints: Computed columns can be used in constraints like CHECK constraints.

Syntax for Creating Computed Columns

Here’s the basic syntax for adding a computed column to a table:

ALTER TABLE TableName
ADD ComputedColumnName AS (Expression) PERSISTED;

Example

Suppose you have a table named Orders with columns for UnitPrice and Quantity, and you want to add a computed column to calculate the total price of each order:

1. Create the Table:

   CREATE TABLE Orders (
       OrderID INT PRIMARY KEY,
       UnitPrice DECIMAL(10, 2),
       Quantity INT
   );

2. Add a Computed Column:

   ALTER TABLE Orders
   ADD TotalPrice AS (UnitPrice * Quantity) PERSISTED;

   In this example:

   • TotalPrice is a computed column that multiplies UnitPrice by Quantity.
   • The PERSISTED keyword indicates that the computed values are stored on disk, which can improve performance.

3. Insert Data and Query:

   INSERT INTO Orders (OrderID, UnitPrice, Quantity)
   VALUES (1, 19.99, 5);
   
   SELECT OrderID, UnitPrice, Quantity, TotalPrice
   FROM Orders;

   The TotalPrice column will automatically show the computed value of UnitPrice * Quantity.

Benefits of Computed Columns

• Data Integrity: Computed columns ensure that derived values are always up-to-date and consistent with the underlying data.
• Reduced Redundancy: They eliminate the need to store redundant data, as values are derived from existing columns.
• Improved Readability: Computed columns can simplify queries by encapsulating complex calculations.

Considerations

• Performance: For non-persisted computed columns, performance may be impacted by the need to compute the value on the fly. Persisted computed columns mitigate this issue but require additional storage.
• Indexing: Indexes on computed columns can enhance query performance, but they also require additional maintenance and storage.

Summary

Computed columns are a powerful feature in SQL Server that allow you to derive values from other columns in a table, improving data consistency and reducing redundancy. They can be configured as persisted or non-persisted, and indexed for performance, making them versatile for various use cases.