How to Add a Default Value to an Existing Column in MariaDB

Here's a breakdown of the steps:

ALTER TABLE <table_name> ALTER COLUMN <column_name> SET DEFAULT <value>;

Replace <table_name> with the actual name of your table and <column_name> with the name of the column you want to modify. <value> represents the default value you want to assign. The value can be a number, text string, or an expression (depending on the column's data type).

1. Execute the ALTER TABLE Statement: Once you've constructed the statement, you can run it in your MariaDB client tool to apply the changes.

Here's an example:

Let's say you have a table named customers with a column named state that currently allows null values. You want to set a default state of 'CA' (California) for new entries.

The SQL statement would be:

ALTER TABLE customers ALTER COLUMN state SET DEFAULT 'CA';

This will modify the state column to have a default value of 'CA'. Any new rows inserted that don't specify a value for state will automatically be populated with 'CA'.

Important Considerations:

• Make sure the default value you choose is compatible with the data type of the column.
• If there's existing data in the column, setting a default value won't affect those existing rows. Their values will remain unchanged.
• In some cases, you may not be able to set certain expressions or functions as default values.

This example sets the default state in the customers table's state column to 'CA':

ALTER TABLE customers ALTER COLUMN state SET DEFAULT 'CA';

This example sets the default value for a new column named points in the users table to 0:

ALTER TABLE users ADD points INT NOT NULL DEFAULT 0;

This example sets the default value for a created_at column in the articles table to the current date:

ALTER TABLE articles ADD created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP;

Example 4: Modifying an existing column with NULL default to a specific value

This example changes the default value for the age column in the employees table from NULL to 25:

ALTER TABLE employees ALTER COLUMN age SET DEFAULT 25;

1. Using a CHECK constraint (limited applicability):

A CHECK constraint allows you to define a condition that all values in a column must adhere to. While not directly setting a default value, you can use it to enforce a specific value for new entries if the column allows nulls.

ALTER TABLE customers ADD CONSTRAINT check_state CHECK (state = 'CA');

This enforces that any new entry in the state column must be 'CA'. However, this approach has limitations:

• It doesn't actually set a default value and won't populate existing null values.
• It can become cumbersome for complex validation rules.

1. Populating null values with an UPDATE statement (for existing data):

If you want to set a default value for existing null values in a column, you can use an UPDATE statement after adding the SET DEFAULT clause with ALTER TABLE.

Here's an example assuming you already set 'CA' as the default for state in the customers table:

UPDATE customers SET state = 'CA' WHERE state IS NULL;

This will update all rows with null values in the state column to 'CA'.