# ALTER TABLE

ALTER TABLE changes the definition of an existing table. CoordinatorDB supports adding, dropping and renaming columns, and renaming the table itself.

## Supported operations

```text
ALTER TABLE table ADD COLUMN column type [constraints]
ALTER TABLE table DROP COLUMN column
ALTER TABLE table RENAME COLUMN old_name TO new_name
ALTER TABLE table RENAME TO new_name
```

A change must remain valid for existing rows and dependent schema objects. Constraints, indexes, views or triggers can prevent a change that would leave them invalid.

## Example

### Adding a defaulted column

```sql
CREATE TABLE accounts(id INTEGER PRIMARY KEY);
INSERT INTO accounts VALUES (1);
ALTER TABLE accounts ADD COLUMN status String DEFAULT 'new';
```

```sql
SELECT id, status
FROM accounts
FORMAT csv;
```

```text
1,"new"
```

See [column operations](https://coordinatordb.com/docs/reference/statements/alter/column) for the detailed syntax and [renaming a table](https://coordinatordb.com/docs/reference/statements/rename) for the rename form. Changing a column's type with `ALTER COLUMN TYPE` or ClickHouse `MODIFY COLUMN` is not supported; create a new table and copy explicitly converted values instead.