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(show you how to set a default value for a column using the DEFAULT constraint)

Introduction to the MySQL DEFAULT constraint

MySQL DEFAULT constraint allows you to specify a default value for a column. Here’s the syntax of the DEFAULT constraint:

column_name data_type DEFAULT default_value;

In this syntax, you specify the DEFAULT keyword followed by the default value for the column. The type of the default value matches the data type of the column.

The default_value must be a literal constant, e.g., a number or a string. It cannot be a function or an expression. However, MySQL allows you to set the current date and time (CURRENT_TIMESTAMP) to the TIMESTAMP and DATETIMEcolumns.

When you define a column without the NOT NULL constraint, the column will implicitly take NULL as the default value.

If a column has a DEFAULT constraint and the INSERT or UPDATE statement doesn’t provide the value for that column, MySQL will use the default value specified in the DEFAULT constraint.

Typically, you set the DEFAULT constraints for columns when you create the table. MySQL also allows you to add default constraints to the columns of existing tables. If you don’t want to use default values for columns, you can remove the default constraints.

1. MySQL DEFAULT constraint example

The following example creates a new table named cart_items with four columns item_id, name, quantity, and sales_tax:

CREATE TABLE cart_items
(
    item_id INT AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(255) NOT NULL,
    quantity INT NOT NULL,
    price DEC(5,2) NOT NULL,
    sales_tax DEC(5,2) NOT NULL DEFAULT 0.1,
    CHECK(quantity > 0),
    CHECK(sales_tax >= 0)
);

NOTE on price DEC(5,2)

The sales_tax column has a default value 0.1 (10%). The following statement shows the cart_items table:

DESC cart_items;

+-----------+--------------+------+-----+---------+----------------+
| Field     | Type         | Null | Key | Default | Extra          |
+-----------+--------------+------+-----+---------+----------------+
| item_id   | int          | NO   | PRI | NULL    | auto_increment |
| name      | varchar(255) | NO   |     | NULL    |                |
| quantity  | int          | NO   |     | NULL    |                |
| price     | decimal(5,2) | NO   |     | NULL    |                |
| sales_tax | decimal(5,2) | NO   |     | 0.10    |                |
+-----------+--------------+------+-----+---------+----------------+
5 rows in set (0.01 sec)

The following INSERT statement adds a new item to the cart_items table:

INSERT INTO cart_items(name, quantity, price)
VALUES('Keyboard', 1, 50);

In this example, the INSERT statement doesn’t provide a value for the sales_tax column. The sales_tax column useS the default value specified in the DEFAULT constraint:

SELECT * FROM cart_items;

Output:

+---------+----------+----------+-------+-----------+
| item_id | name     | quantity | price | sales_tax |
+---------+----------+----------+-------+-----------+
|       1 | Keyboard |        1 | 50.00 |      0.10 |
+---------+----------+----------+-------+-----------+
1 row in set (0.00 sec)

Also, you can explicitly use the DEFAULT keyword when you insert a new row into the cart_items table:

INSERT INTO cart_items(name, quantity, price, sales_tax)
VALUES('Battery',4, 0.25 , DEFAULT);

In this case, the sales_tax column takes the default value:

SELECT * FROM cart_items;

Output:

+---------+----------+----------+-------+-----------+
| item_id | name     | quantity | price | sales_tax |
+---------+----------+----------+-------+-----------+
|       1 | Keyboard |        1 | 50.00 |      0.10 |
|       2 | Battery  |        4 |  0.25 |      0.10 |
+---------+----------+----------+-------+-----------+
2 rows in set (0.01 sec)

2. Adding a DEFAULT constraint to a column

To add a default constraint to a column of an existing table, you use the ALTER TABLE statement:

ALTER TABLE table_name
ALTER COLUMN column_name SET DEFAULT default_value;

The following example adds a DEFAULT constraint to the quantity column of the cart_itesm table:

ALTER TABLE cart_items
ALTER COLUMN quantity SET DEFAULT 1;

If you describe the cart_items table, you’ll see the changes:

DESC cart_items;Code language: SQL (Structured Query Language) (sql)

Output:

+-----------+--------------+------+-----+---------+----------------+
| Field     | Type         | Null | Key | Default | Extra          |
+-----------+--------------+------+-----+---------+----------------+
| item_id   | int          | NO   | PRI | NULL    | auto_increment |
| name      | varchar(255) | NO   |     | NULL    |                |
| quantity  | int          | NO   |     | 1       |                |
| price     | decimal(5,2) | NO   |     | NULL    |                |
| sales_tax | decimal(5,2) | NO   |     | 0.10    |                |
+-----------+--------------+------+-----+---------+----------------+
5 rows in set (0.00 sec)

he following statement inserts a new row into the cart_itemstable without specifying a value for the quantity column:

INSERT INTO cart_items(name, price, sales_tax)
VALUES('Maintenance services',25.99, 0)

The value of the quantity column will default to 1:

SELECT * FROM cart_items;

+---------+----------------------+----------+-------+-----------+
| item_id | name                 | quantity | price | sales_tax |
+---------+----------------------+----------+-------+-----------+
|       1 | Keyboard             |        1 | 50.00 |      0.10 |
|       2 | Battery              |        4 |  0.25 |      0.10 |
|       3 | Maintenance services |        1 | 25.99 |      0.00 |
+---------+----------------------+----------+-------+-----------+
3 rows in set (0.00 sec)

3. Removing a DEFAULT constraint from a column

To remove a DEFAULT constraint from a column, you use the ALTER TABLE statement:

ALTER TABLE table_name
ALTER column_name DROP DEFAULT;Code language: SQL (Structured Query Language) (sql)

The following example removes the DEFAULT constraint from the quantity column of the cart_items table:

ALTER TABLE cart_items
ALTER COLUMN quantity DROP DEFAULT;Code language: SQL (Structured Query Language) (sql)

And here’s the new cart_items structure:

DESC cart_items;Code language: SQL (Structured Query Language) (sql)

Output:

+-----------+--------------+------+-----+---------+----------------+
| Field     | Type         | Null | Key | Default | Extra          |
+-----------+--------------+------+-----+---------+----------------+
| item_id   | int          | NO   | PRI | NULL    | auto_increment |
| name      | varchar(255) | NO   |     | NULL    |                |
| quantity  | int          | NO   |     | NULL    |                |
| price     | decimal(5,2) | NO   |     | NULL    |                |
| sales_tax | decimal(5,2) | NO   |     | 0.10    |                |
+-----------+--------------+------+-----+---------+----------------+
5 rows in set (0.00 sec)

Summary

MySQL DEFAULT constraints set default values for columns.
Use DEFAULT default_value to set a default constraint to a column.
Use ALTER TABLE ... ALTER COLUMN ... SET DEFAULT to add a DEFAULT constraint to a column of an existing table.
Use ALTER TABLE ... ALTER COLUMN ... DROP DEFAULT to drop a DEFAULT constraint from a column of an existing table.