What does default value in mysql mean?

Default values ​​in MySQL are predefined values ​​automatically assigned by the database when an insert operation does not specify a column value. They are used to ensure data consistency, simplify data entry, and prevent invalid/null values. Default values ​​can be set through the DEFAULT keyword when creating a table or column, and types such as constant values, expressions, and NULL are supported. MySQL also provides special default values ​​such as NULL (for a null value), CURRENT_TIMESTAMP (for inserting a timestamp), CURRENT_DATE (for inserting a date), and CURRENT_USER (for inserting a user name).

Default value in MySQL

In the MySQL database, the default value means that if there is no Specify the value of a column, and the database will automatically assign a predefined value to this column.

Use of default values

Default values ​​are usually used in the following scenarios:

• Ensure the consistency of data in the table, such as creation date , status or user type.
• Simplify data entry and avoid manually specifying values ​​for non-null columns.
• Prevent the insertion of invalid or null values ​​and improve data quality.

Set the default value

You can set the default value by using the DEFAULT keyword when creating a table or column, for example:

<code class="sql">CREATE TABLE users (
  id INT NOT NULL AUTO_INCREMENT,
  username VARCHAR(255) NOT NULL,
  email VARCHAR(255) NOT NULL DEFAULT 'example@domain.com'
);</code>

In this example, the default value for the email column is set to 'example@domain.com'.

Default value types

MySQL supports various default value types, including:

• Constant values ​​(such as strings, numbers, or dates )
• Expression (such as calculating the current date or time)
• NULL (indicates a null value)

Special default value

MySQL also provides several special default values:

• NULL: Indicates a null value.
• CURRENT_TIMESTAMP: The current timestamp when the record was inserted.
• CURRENT_DATE: The current date when inserting the record.
• CURRENT_USER: The current user name when inserting records.

When using these special default values, there is no need to specify any value, the database will automatically generate the value based on the current situation.

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