Default Value Constraint

The DEFAULT value specifies a value to write into the constrained column if one is not defined in an INSERT statement. The value may be either a hard-coded literal or an expression that is evaluated at the time the row is created.

Details

• The of the DEFAULT value must be the same as the data type of the column.
• The DEFAULT value constraint only applies if the column does not have a value specified in the statement. You can still insert a NULL into an optional (nullable) column by explicitly inserting NULL. For example, INSERT INTO foo VALUES (1, NULL);.

Syntax

You can only apply the DEFAULT value constraint to individual columns.

You can also add the DEFAULT value constraint to an existing table through .

Parameter	Description
table_name	The name of the table you’re creating.
column_name	The name of the constrained column.
column_type	The constrained column’s .
default_value	The value you want to insert by default, which must evaluate to the same as the column_type.
column_constraints	Any other column-level you want to apply to this column.
column_def	Definitions for any other columns in the table.
table_constraints	Any table-level you want to apply.

Example

> CREATE TABLE inventories (
    product_id        INT,
    warehouse_id      INT,
    quantity_on_hand  INT DEFAULT 100,
    PRIMARY KEY (product_id, warehouse_id)
  );

> INSERT INTO inventories (product_id, warehouse_id) VALUES (1,20);

> INSERT INTO inventories (product_id, warehouse_id, quantity_on_hand) VALUES (2,30, NULL);

> SELECT * FROM inventories;

+------------+--------------+------------------+
| product_id | warehouse_id | quantity_on_hand |
+------------+--------------+------------------+
|          1 |           20 |              100 |
|          2 |           30 | NULL             |
+------------+--------------+------------------+

If the DEFAULT value constraint is not specified and an explicit value is not given, a value of NULL is assigned to the column.

See also