# Identity columns
<a name="sequences-identity-columns-overview"></a>

**Important**  
When using identity columns, the cache value should be carefully considered. For more information, see the Important callout on the [`CREATE SEQUENCE`](create-sequence-syntax-support.md) page.  
For guidance on how best to use identity columns based on workload patterns, see [Working with sequences and identity columns](sequences-identity-columns-working-with.md).

An identity column is a special column that's generated automatically from an implicit sequence. It can be used to generate key values. To create an identity column, use the `GENERATED ... AS IDENTITY` clause in [`CREATE TABLE`](create-table-syntax-support.md), for example:

```
CREATE TABLE people (
    id bigint GENERATED ALWAYS AS IDENTITY (CACHE 70000),
    ...
);
```

or alternatively:

```
CREATE TABLE people (
    id bigint GENERATED BY DEFAULT AS IDENTITY (CACHE 70000),
    ...
);
```

See [`CREATE TABLE`](create-table-syntax-support.md) for more details.

If an `INSERT` command is executed on the table with the identity column and no value is explicitly specified for the identity column, then a value generated by the implicit sequence is inserted. For example, with the preceding definitions and assuming additional appropriate columns, writing:

```
INSERT INTO people (name, address) VALUES ('A', 'foo');
INSERT INTO people (name, address) VALUES ('B', 'bar');
```

would generate values for the `id` column starting at 1 and result in the following table data:

```
 id | name | address
----+------+---------
  1 | A    | foo
  2 | B    | bar
```

Alternatively, the keyword `DEFAULT` can be specified in place of a value to explicitly request the sequence-generated value:

```
INSERT INTO people (id, name, address) VALUES (DEFAULT, 'C', 'baz');
```

Similarly, the keyword `DEFAULT` can be used in `UPDATE` commands.

Thus, in many ways, an identity column behaves like a column with a default value.

The clauses `ALWAYS` and `BY DEFAULT` in the column definition determine how explicitly user-specified values are handled in `INSERT` and `UPDATE` commands. In an `INSERT` command, if `ALWAYS` is selected, a user-specified value is only accepted if the `INSERT` statement specifies `OVERRIDING SYSTEM VALUE`. If `BY DEFAULT` is selected, then the user-specified value takes precedence. Thus, using `BY DEFAULT` results in a behavior more similar to default values, where the default value can be overridden by an explicit value, whereas `ALWAYS` provides some more protection against accidentally inserting an explicit value.

The data type of an identity column must be one of the data types supported by sequences. (See [`CREATE SEQUENCE`](create-sequence-syntax-support.md).) The properties of the associated sequence might be specified when creating an identity column (see [`CREATE TABLE`](create-table-syntax-support.md)) or changed afterwards (see [`ALTER TABLE`](alter-table-syntax-support.md)).

An identity column is automatically marked as `NOT NULL`. An identity column, however, doesn't guarantee uniqueness. (A sequence normally returns unique values, but a sequence could be reset, or values could be inserted manually into the identity column, as discussed previously.) Uniqueness would need to be enforced using a `PRIMARY KEY` or `UNIQUE` constraint.