# ALTER TABLE columns

Column operations change a table's schema and validate the change against existing rows and dependencies.

## Syntax

```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
```

## Adding a column

An added column must have an allowed definition that also works for rows already in the table. Supply a suitable DEFAULT when existing rows need a value, particularly with `NOT NULL`.

## Dropping or renaming a column

Dependent constraints, indexes, views and triggers can prevent an incompatible change. Renaming a column changes its schema name; use the new name in later statements.

`ALTER COLUMN TYPE` and ClickHouse `MODIFY COLUMN` are not supported. To change the value type, create a replacement table and use `INSERT ... SELECT CAST(...)`.

## Examples

### Adding and renaming a field

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

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

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

### Dropping an unused field

```sql
ALTER TABLE profiles ADD COLUMN note String;
ALTER TABLE profiles DROP COLUMN note;
```

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

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

See [`CREATE TABLE`](https://coordinatordb.com/docs/reference/statements/create/table) for column definitions and constraints.