How to Add Default Constraint in SQL Using Alter
Adding a default constraint to a column in SQL is a crucial step in ensuring data integrity and consistency within a database. Default constraints automatically assign a value to a column when no value is provided during an insert operation. This article will guide you through the process of adding a default constraint to a column using the ALTER TABLE statement in SQL.
Understanding Default Constraints
Before diving into the specifics of adding a default constraint, it’s essential to understand what a default constraint is and how it works. A default constraint is a predefined value that is automatically assigned to a column if no explicit value is provided by the user. This predefined value is specified in the column definition and is applied whenever a new row is inserted into the table.
Adding a Default Constraint Using ALTER TABLE
To add a default constraint to an existing column in SQL, you can use the ALTER TABLE statement along with the ADD CONSTRAINT clause. The following steps outline the process:
1. Identify the table and column to which you want to add the default constraint.
2. Determine the default value you want to assign to the column.
3. Use the ALTER TABLE statement to add the default constraint.
Here’s an example of how to add a default constraint to a column named “age” in a table called “employees”:
“`sql
ALTER TABLE employees
ADD CONSTRAINT df_age
DEFAULT 18
FOR age;
“`
In this example, the default value of 18 is assigned to the “age” column whenever a new row is inserted without specifying an age value.
Verifying the Default Constraint
After adding the default constraint, it’s important to verify that it has been applied correctly. You can do this by querying the database schema or by attempting to insert a new row without specifying a value for the constrained column.
To verify the default constraint, you can use the following SQL query:
“`sql
SELECT
FROM information_schema.table_constraints
WHERE constraint_name = ‘df_age’;
“`
This query will return information about the default constraint you just added, including the table name, column name, and the default value.
Conclusion
Adding a default constraint to a column in SQL using the ALTER TABLE statement is a straightforward process that ensures data consistency and integrity. By following the steps outlined in this article, you can easily add a default constraint to an existing column and verify its application. Remember to choose a default value that makes sense for your specific use case and test the constraint to ensure it behaves as expected.
