The ALTER TABLE statement changes the schema of an existing table in the catalog.

Examples

CREATE TABLE integers (i INTEGER, j INTEGER);

Add a new column with name k to the table integers, it will be filled with the default value NULL:

ALTER TABLE integers
ADD COLUMN k INTEGER;

Add a new column with name l to the table integers, it will be filled with the default value 10:

ALTER TABLE integers
ADD COLUMN l INTEGER DEFAULT 10;

Drop the column k from the table integers:

ALTER TABLE integers
DROP k;

Change the type of the column i to the type VARCHAR using a standard cast:

ALTER TABLE integers
ALTER i TYPE VARCHAR;

Change the type of the column i to the type VARCHAR, using the specified expression to convert the data for each row:

ALTER TABLE integers
ALTER i SET DATA TYPE VARCHAR USING concat(i, '_', j);

Set the default value of a column:

ALTER TABLE integers
ALTER COLUMN i SET DEFAULT 10;

Drop the default value of a column:

ALTER TABLE integers
ALTER COLUMN i DROP DEFAULT;

Make a column not nullable:

ALTER TABLE integers
ALTER COLUMN i SET NOT NULL;

Drop the not-NULL constraint:

ALTER TABLE integers
ALTER COLUMN i DROP NOT NULL;

Rename a table:

ALTER TABLE integers
RENAME TO integers_old;

Rename a column of a table:

ALTER TABLE integers
RENAME i TO ii;

Add a primary key to a column of a table:

ALTER TABLE integers
ADD PRIMARY KEY (i);

Syntax

ALTER TABLE changes the schema of an existing table. All the changes made by ALTER TABLE fully respect the transactional semantics, i.e., they will not be visible to other transactions until committed, and can be fully reverted through a rollback.

RENAME TABLE

Rename a table:

ALTER TABLE integers
RENAME TO integers_old;

The RENAME TO clause renames an entire table, changing its name in the schema. Note that any views that rely on the table are not automatically updated.

RENAME COLUMN

Rename a column of a table:

ALTER TABLE integers 
RENAME i TO j;

ALTER TABLE integers
RENAME COLUMN j TO k;

The RENAME COLUMN clause renames a single column within a table. Any constraints that rely on this name (e.g., CHECK constraints) are automatically updated. However, note that any views that rely on this column name are not automatically updated.

ADD COLUMN

Add a new column with name k to the table integers, it will be filled with the default value NULL:

ALTER TABLE integers
ADD COLUMN k INTEGER;

Add a new column with name l to the table integers, it will be filled with the default value 10:

ALTER TABLE integers
ADD COLUMN l INTEGER DEFAULT 10;

The ADD COLUMN clause can be used to add a new column of a specified type to a table. The new column will be filled with the specified default value, or NULL if none is specified.

DROP COLUMN

Drop the column k from the table integers:

ALTER TABLE integers
DROP k;

The DROP COLUMN clause can be used to remove a column from a table. Note that columns can only be removed if they do not have any indexes that rely on them. This includes any indexes created as part of a PRIMARY KEY or UNIQUE constraint. Columns that are part of multi-column check constraints cannot be dropped either. If you attempt to drop a column with an index on it, DuckDB will return the following error message:

Dependency Error:
Cannot alter entry "..." because there are entries that depend on it.

[SET [DATA]] TYPE

Change the type of the column i to the type VARCHAR using a standard cast:

ALTER TABLE integers
ALTER i TYPE VARCHAR;

Instead of ALTER column_name TYPE type, you can also use the equivalent ALTER column_name SET TYPE type and the ALTER column_name SET DATA TYPE type clauses.

Change the type of the column i to the type VARCHAR, using the specified expression to convert the data for each row:

ALTER TABLE integers
ALTER i SET DATA TYPE VARCHAR USING concat(i, '_', j);

The [SET [DATA]] TYPE clause changes the type of a column in a table. Any data present in the column is converted according to the provided expression in the USING clause, or, if the USING clause is absent, cast to the new data type. Note that columns can only have their type changed if they do not have any indexes that rely on them and are not part of any CHECK constraints.

To change the sub-schema of a STRUCT-typed column, use ALTER TABLE with the struct_insert function. For example:

CREATE TABLE tbl (col STRUCT(i INTEGER));
ALTER TABLE tbl
ALTER col TYPE USING struct_insert(col, a := 42, b := NULL::VARCHAR);

SET / DROP DEFAULT

Set the default value of a column:

ALTER TABLE integers
ALTER COLUMN i SET DEFAULT 10;

Drop the default value of a column:

ALTER TABLE integers
ALTER COLUMN i DROP DEFAULT;

The SET/DROP DEFAULT clause modifies the DEFAULT value of an existing column. Note that this does not modify any existing data in the column. Dropping the default is equivalent to setting the default value to NULL.

Warning At the moment DuckDB will not allow you to alter a table if there are any dependencies. That means that if you have an index on a column you will first need to drop the index, alter the table, and then recreate the index. Otherwise, you will get a Dependency Error.

ADD PRIMARY KEY

Add a primary key to a column of a table:

ALTER TABLE integers
ADD PRIMARY KEY (i);

Add a primary key to multiple columns of a table:

ALTER TABLE integers
ADD PRIMARY KEY (i, j);

ADD / DROP CONSTRAINT

ADD CONSTRAINT and DROP CONSTRAINT clauses are not yet supported in DuckDB.

Limitations

ALTER COLUMN fails if values of conflicting types have occurred in the table at any point, even if they have been deleted:

CREATE TABLE tbl (col VARCHAR);

INSERT INTO tbl
VALUES ('asdf'), ('42');

DELETE FROM tbl
WHERE col = 'asdf';

ALTER TABLE tbl
ALTER COLUMN col TYPE INTEGER;

Conversion Error:
Could not convert string 'asdf' to INT32

Currently, this is expect behavior. As a workaround, you can create a copy of the table:

CREATE OR REPLACE TABLE tbl AS FROM tbl;