SQL Default

In SQL, the DEFAULT constraint is used to specify a default value for a column. If a value is not specified for the column when a new row is inserted, the default value will be used instead.

Here is an example of how to create a table with a DEFAULT constraint:

CREATE TABLE employees (
    employee_id INT PRIMARY KEY,
    employee_name VARCHAR(50),
    hire_date DATE DEFAULT GETDATE(),
    salary DECIMAL(10,2) DEFAULT 50000.00
);

In this example, the employees table has a DEFAULT constraint on the hire_date and salary columns. If a value is not specified for the hire_date column, the default value will be the current date and time. If a value is not specified for the salary column, the default value will be 50000.00.

You can also use the DEFAULT constraint to specify a column's default value as a function of other columns in the same row. For example:

CREATE TABLE orders (
    order_id INT PRIMARY KEY,
    customer_id INT,
    order_date DATE DEFAULT GETDATE(),
    order_total DECIMAL(10,2),
    discount DECIMAL(4,2) DEFAULT 0.0,
    total_after_discount AS (order_total - (order_total * discount))
);

In this example, the orders table has a DEFAULT constraint on the discount column. If a value is not specified for the discount column, the default value will be 0.0. The total_after_discount column is computed as a function of the order_total and discount columns.

You can modify the DEFAULT constraint for an existing table using the ALTER TABLE command:

ALTER TABLE employees
ALTER COLUMN hire_date SET DEFAULT GETDATE();

In this example, we are modifying the DEFAULT constraint for the hire_date column of the employees table using the ALTER TABLE command.

By using the DEFAULT constraint, you can ensure that a column always has a value, even if a value is not specified for it when a new row is inserted. This can help simplify data entry and ensure that data is consistent across multiple rows in the table.