Question
Answer and Explanation
Altering a regular column to a computed column in Microsoft SQL Server (MSSQL) involves a few steps. The process requires you to drop the existing column and then add it back as a computed column. Here’s how you can do it:
1. Drop the existing column: First, you need to remove the original column. Use the `ALTER TABLE` statement with the `DROP COLUMN` clause. For instance, if you have a column named `RegularColumn` in a table named `YourTable`, the SQL would be:
ALTER TABLE YourTable
DROP COLUMN RegularColumn;
2. Add the column back as a computed column: Next, you re-add the column using `ALTER TABLE` statement with `ADD` clause and the `AS` keyword for specifying the computed column definition. The syntax will look like this:
ALTER TABLE YourTable
ADD RegularColumn AS (Expression);
- Here, `Expression` is the formula or logic that defines the computed column. For example, if you want to calculate the product of two columns named `ColumnA` and `ColumnB`, the SQL would be:
ALTER TABLE YourTable
ADD RegularColumn AS (ColumnA ColumnB);
3. Complete Example: Here’s a complete example putting it all together:
-- Assuming the existing table is named 'YourTable'
-- And you have an existing column named 'RegularColumn'
-- Step 1: Drop the existing column
ALTER TABLE YourTable
DROP COLUMN RegularColumn;
-- Step 2: Add the computed column with the desired expression
ALTER TABLE YourTable
ADD RegularColumn AS (ColumnA ColumnB);
4. Important Considerations:
- Make sure the `Expression` uses other columns from the same table. - Computed columns do not store their values physically in the table, they are calculated on the fly when queried. - You cannot directly insert or update values in computed columns. - Any queries using this computed column will execute the expression on demand, which might have performance implications if complex. - If the underlying columns change, the computed column's result will automatically update.
By following these steps, you can successfully alter an existing column into a computed column within MSSQL. Always back up your database before making such schema changes.