A computed column is a virtual column in a database table that is not physically stored on the disk (unless you specifically configure it for storage). Instead, its value is calculated using an expression involving other columns in the same table whenever it is queried. Computed columns can be very useful for simplifying queries and enhancing performance by reducing the need for complex calculations to be performed repeatedly.
How to Create a Computed Column
The syntax for creating a computed column can vary slightly depending on the database management system (DBMS) being used. Below are examples for SQL Server and MySQL.
SQL Server Example
In SQL Server, you can create a computed column as follows:
CREATE TABLE Employees (
EmployeeID INT PRIMARY KEY,
FirstName NVARCHAR(50),
LastName NVARCHAR(50),
FullName AS (FirstName + ' ' + LastName) -- This is the computed column
);
You can also use expressions and functions in your computed column declaration:
CREATE TABLE Orders (
OrderID INT PRIMARY KEY,
Quantity INT,
UnitPrice DECIMAL(10, 2),
TotalPrice AS (Quantity * UnitPrice) -- Computed column for total price
);
You can also specify whether the computed column should be persisted (stored on disk):
TotalPrice AS (Quantity * UnitPrice) PERSISTED
MySQL Example
In MySQL, you can create a generated (computed) column using the following syntax:
CREATE TABLE Employees (
EmployeeID INT PRIMARY KEY,
FirstName VARCHAR(50),
LastName VARCHAR(50),
FullName VARCHAR(101) AS (CONCAT(FirstName, ' ', LastName)) STORED -- This is a generated column
);
In the MySQL example, you can specify whether to store the generated column physically (using STORED) or compute it on the fly (using VIRTUAL, which is the default).
Benefits of Computed Columns
1. Ease of Use: Simplifies queries by encapsulating complex calculations within a column, allowing users to access computed results without performing recalculations.
2. Consistency: Ensures consistent calculation across multiple queries. Any change in the underlying data automatically updates the computed column value, reducing the chance of errors.
3. Performance: Can improve query performance, particularly when the computed result is used frequently. When a computed column is persisted, it can help reduce computation time when accessing data.
4. Readability: Improves the readability of SQL queries by providing descriptive column names for calculated values, making it clearer for others who read the queries.
5. Indexing: In some DBMSs, computed columns can be indexed, which can enhance performance for certain types of queries.
6. Data Integrity: Helps to enforce business rules and calculations at the database level, ensuring that the data adheres to specified rules without requiring additional application logic.
In conclusion, computed columns are a powerful feature in relational databases that can streamline data management, improve performance, and enhance the clarity of database queries.