How to Alter Table in SQL with Default Value
In SQL, altering a table to include a default value is a common task that can be achieved using the ALTER TABLE statement. This statement allows you to modify the structure of an existing table by adding, modifying, or dropping columns. In this article, we will explore how to alter a table in SQL to set a default value for a column.
Understanding Default Values
Before diving into the syntax, it’s important to understand what a default value is. A default value is a predefined value that is automatically assigned to a column when a new row is inserted into the table, and no value is specified for that column. This can be particularly useful for ensuring data consistency and simplifying the insertion process.
Example Scenario
Let’s consider a scenario where we have a table named “employees” with the following columns: “employee_id” (INT), “first_name” (VARCHAR), “last_name” (VARCHAR), and “department” (VARCHAR). We want to add a new column called “hire_date” (DATE) with a default value of the current date.
Using the ALTER TABLE Statement
To add the “hire_date” column with a default value, we will use the following syntax:
“`sql
ALTER TABLE employees
ADD COLUMN hire_date DATE DEFAULT CURRENT_DATE;
“`
In this example, the ALTER TABLE statement is used to add a new column to the “employees” table. The ADD COLUMN clause specifies the name of the column to be added, followed by its data type. The DEFAULT keyword is used to set the default value for the column, and CURRENT_DATE is a built-in function that returns the current date.
Modifying an Existing Column’s Default Value
If you want to modify the default value of an existing column, you can use a similar approach. Let’s say we want to change the default value of the “hire_date” column in the “employees” table to a specific date, such as ‘2023-01-01’.
“`sql
ALTER TABLE employees
ALTER COLUMN hire_date SET DEFAULT ‘2023-01-01’;
“`
In this case, the ALTER COLUMN clause is used to modify the default value of the “hire_date” column. The SET keyword is followed by the new default value.
Conclusion
In this article, we discussed how to alter a table in SQL to set a default value for a column. By using the ALTER TABLE statement with the ADD COLUMN and ALTER COLUMN clauses, you can easily add or modify default values to ensure data consistency and simplify the insertion process. Remember to consider the data type and constraints of your table when setting default values to avoid any potential issues.
