Mastering SQL- How to Set Default Values for Columns with ALTER COLUMN
How to Set Default Value in SQL Alter Column
In SQL, setting a default value for a column is a crucial aspect of database design. It ensures that every row in the table has a predefined value for that column if no other value is specified during the insertion or update of the row. This can be particularly useful for columns that are not required to be filled in by the user, such as timestamps or default categories. In this article, we will discuss how to set a default value in SQL using the ALTER COLUMN statement.
Understanding the ALTER COLUMN Statement
The ALTER COLUMN statement is used to modify the properties of a column in an existing table. This includes changing the data type, adding or removing constraints, and, of course, setting a default value. To set a default value for a column, you need to use the SET DEFAULT clause within the ALTER COLUMN statement.
Example of Setting a Default Value
Let’s consider an example where we have a table named “employees” with a column named “department_id.” We want to set the default value for this column to “HR” (Human Resources) for all new rows that are inserted into the table.
“`sql
ALTER TABLE employees
ALTER COLUMN department_id SET DEFAULT ‘HR’;
“`
In this example, the ALTER TABLE statement is used to modify the “employees” table. The ALTER COLUMN clause specifies that we want to change the properties of the “department_id” column. By using the SET DEFAULT ‘HR’ clause, we are setting the default value for this column to “HR.”
Checking the Default Value
After setting the default value, it’s essential to verify that the change has been applied correctly. You can do this by querying the table structure or by inserting a new row and checking the default value for the “department_id” column.
“`sql
— Query the table structure
DESCRIBE employees;
— Insert a new row and check the default value
INSERT INTO employees (name, department_id) VALUES (‘John Doe’, NULL);
SELECT FROM employees WHERE name = ‘John Doe’;
“`
In the above queries, the DESCRIBE statement is used to display the structure of the “employees” table, including the default value for the “department_id” column. The INSERT INTO statement is used to insert a new row with a NULL value for the “department_id” column, and the SELECT statement retrieves the inserted row to confirm that the default value “HR” has been assigned.
Conclusion
Setting a default value for a column in SQL using the ALTER COLUMN statement is a straightforward process. By following the steps outlined in this article, you can ensure that your database maintains consistency and efficiency by automatically assigning predefined values to columns that do not require user input.
