How to Alter a Computed Column in SQL Server
Computing columns in SQL Server are a powerful feature that allows you to store the result of an expression directly in the table. This can save space and improve performance by avoiding the need to store redundant data. However, there may be situations where you need to alter a computed column after it has been created. In this article, we will discuss how to alter a computed column in SQL Server.
First, it’s important to note that computed columns can only be altered if they are not part of a clustered index. If the computed column is part of a clustered index, you will need to drop the index first, alter the computed column, and then recreate the index.
To alter a computed column, you can use the following steps:
1. Open SQL Server Management Studio (SSMS) and connect to your database.
2. In the Object Explorer, navigate to the table that contains the computed column you want to alter.
3. Right-click on the table and select “Design” to open the table designer.
4. In the table designer, locate the computed column you want to alter.
5. Double-click on the computed column to open the “Computed Column Properties” dialog box.
6. In the “Expression” box, modify the expression to reflect the new computation you want to apply to the column.
7. Click “OK” to save the changes.
After you have altered the computed column, you may need to update the data in the column to reflect the new computation. This can be done by running an update statement that applies the new expression to the column.
Here’s an example of how to alter a computed column:
“`sql
— Assume we have a table named ‘Employees’ with a computed column ‘Age’ based on the ‘BirthDate’ column.
— Step 1: Open the table designer for the ‘Employees’ table.
— Step 2: Double-click on the ‘Age’ computed column.
— Step 3: Modify the expression to calculate the age based on the current date.
ALTER TABLE Employees
ALTER COLUMN Age AS (DATEDIFF(year, BirthDate, GETDATE()) – CASE WHEN (MONTH(BirthDate) > MONTH(GETDATE())) OR (MONTH(BirthDate) = MONTH(GETDATE()) AND DAY(BirthDate) > DAY(GETDATE())) THEN 1 ELSE 0 END);
“`
In this example, we have changed the expression for the ‘Age’ computed column to calculate the age by subtracting the birth date from the current date, taking into account whether the person has already had their birthday this year.
Remember that altering a computed column can have implications for the data integrity and performance of your database. Always test your changes in a development environment before applying them to a production database.
