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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
DEFAULTvalue constraint only applies if the column does not have a value specified in theINSERTstatement. 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.
| 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 data type. |
default_value | The value you want to insert by default, which must evaluate to the same data type as the column_type. |
column_constraints | Any other column-level constraints you want to apply to this column. |
column_def | Definitions for any other columns in the table. |
table_constraints | Any table-level constraints 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.