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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 Default Value constraint only applies if the column does not have a value specified in the
INSERTstatement. 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. |
Usage 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
- Foreign Key constraint
- Not Null constraint
- Primary Key constraint
- Unique constraint
SHOW CONSTRAINTS
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