The ALTER COLUMN statement is part of ALTER TABLE and sets, changes, or drops a column's DEFAULT constraint or drops the NOT NULL constraint.
To manage other constraints, see ADD CONSTRAINT and DROP CONSTRAINT.
Synopsis
Required privileges
The user must have the CREATE privilege on the table.
Parameters
| Parameter | Description |
|---|---|
table_name |
The name of the table with the column you want to modify. |
column_name |
The name of the column you want to modify. |
a_expr |
The new Default Value you want to use. |
Viewing schema changes
Whenever you initiate a schema change, CockroachDB registers it as a job, which you can view with SHOW JOBS.
Examples
Set or change a DEFAULT value
Setting the DEFAULT value constraint inserts the value when data's written to the table without explicitly defining the value for the column. If the column already has a DEFAULT value set, you can use this statement to change it.
The below example inserts the Boolean value true whenever you inserted data to the subscriptions table without defining a value for the newsletter column.
> ALTER TABLE subscriptions ALTER COLUMN newsletter SET DEFAULT true;
Remove DEFAULT constraint
If the column has a defined DEFAULT value, you can remove the constraint, which means the column will no longer insert a value by default if one is not explicitly defined for the column.
> ALTER TABLE subscriptions ALTER COLUMN newsletter DROP DEFAULT;
Remove NOT NULL constraint
If the column has the NOT NULL constraint applied to it, you can remove the constraint, which means the column becomes optional and can have NULL values written into it.
> ALTER TABLE subscriptions ALTER COLUMN newsletter DROP NOT NULL;
Convert a computed column into a regular column
New in v2.1: You can convert a stored, computed column into a regular column by using ALTER TABLE.
In this example, create a simple table with a computed column:
> CREATE TABLE office_dogs (
id INT PRIMARY KEY,
first_name STRING,
last_name STRING,
full_name STRING AS (CONCAT(first_name, ' ', last_name)) STORED
);
Then, insert a few rows of data:
> INSERT INTO office_dogs (id, first_name, last_name) VALUES
(1, 'Petee', 'Hirata'),
(2, 'Carl', 'Kimball'),
(3, 'Ernie', 'Narayan');
> SELECT * FROM office_dogs;
+----+------------+-----------+---------------+
| id | first_name | last_name | full_name |
+----+------------+-----------+---------------+
| 1 | Petee | Hirata | Petee Hirata |
| 2 | Carl | Kimball | Carl Kimball |
| 3 | Ernie | Narayan | Ernie Narayan |
+----+------------+-----------+---------------+
(3 rows)
The full_name column is computed from the first_name and last_name columns without the need to define a view. You can view the column details with the SHOW COLUMNS statement:
> SHOW COLUMNS FROM office_dogs;
+-------------+-----------+-------------+----------------+------------------------------------+-------------+
| column_name | data_type | is_nullable | column_default | generation_expression | indices |
+-------------+-----------+-------------+----------------+------------------------------------+-------------+
| id | INT | false | NULL | | {"primary"} |
| first_name | STRING | true | NULL | | {} |
| last_name | STRING | true | NULL | | {} |
| full_name | STRING | true | NULL | concat(first_name, ' ', last_name) | {} |
+-------------+-----------+-------------+----------------+------------------------------------+-------------+
(4 rows)
Now, convert the computed column (full_name) to a regular column:
> ALTER TABLE office_dogs ALTER COLUMN full_name DROP STORED;
Check that the computed column was converted:
> SHOW COLUMNS FROM office_dogs;
+-------------+-----------+-------------+----------------+-----------------------+-------------+
| column_name | data_type | is_nullable | column_default | generation_expression | indices |
+-------------+-----------+-------------+----------------+-----------------------+-------------+
| id | INT | false | NULL | | {"primary"} |
| first_name | STRING | true | NULL | | {} |
| last_name | STRING | true | NULL | | {} |
| full_name | STRING | true | NULL | | {} |
+-------------+-----------+-------------+----------------+-----------------------+-------------+
(4 rows)
The computed column is now a regular column and can be updated as such:
> INSERT INTO office_dogs (id, first_name, last_name, full_name) VALUES (4, 'Lola', 'McDog', 'This is not computed');
> SELECT * FROM office_dogs;
+----+------------+-----------+----------------------+
| id | first_name | last_name | full_name |
+----+------------+-----------+----------------------+
| 1 | Petee | Hirata | Petee Hirata |
| 2 | Carl | Kimball | Carl Kimball |
| 3 | Ernie | Narayan | Ernie Narayan |
| 4 | Lola | McDog | This is not computed |
+----+------------+-----------+----------------------+
(4 rows)
See also
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