Default Value Constraint

The DEFAULT value constraint 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 data type 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 INSERT 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.

Note:

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

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

• Constraints
• ALTER COLUMN
• CHECK constraint
• REFERENCES constraint (Foreign Key)
• NOT NULL constraint
• PRIMARY KEY constraint
• UNIQUE constraint
• SHOW CONSTRAINTS

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