> For the complete documentation index, see [llms.txt](https://docs.postgresql.tw/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.postgresql.tw/the-sql-language/ddl/ddl-identity-columns.md).

# 5.3. Identity Columns \#

An identity column is a special column that is 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`, for example:

```

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

or alternatively

```

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

See [CREATE TABLE](/reference/sql-commands/sql-createtable.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 above 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, like

```

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](/reference/sql-commands/sql-createsequence.md).) The properties of the associated sequence may be specified when creating an identity column (see [CREATE TABLE](/reference/sql-commands/sql-createtable.md)) or changed afterwards (see [ALTER TABLE](/reference/sql-commands/sql-altertable.md)).

An identity column is automatically marked as `NOT NULL`. An identity column, however, does not 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 above.) Uniqueness would need to be enforced using a `PRIMARY KEY` or `UNIQUE` constraint.

In table inheritance hierarchies, identity columns and their properties in a child table are independent of those in its parent tables. A child table does not inherit identity columns or their properties automatically from the parent. During `INSERT` or `UPDATE`, a column is treated as an identity column if that column is an identity column in the table named in the statement, and the corresponding identity properties are applied.

Partitions inherit identity columns from the partitioned table. They cannot have their own identity columns. The properties of a given identity column are consistent across all the partitions in the partition hierarchy.

***

原文：[PostgreSQL 18.6 Documentation](https://www.postgresql.org/docs/18/ddl-identity-columns.html)（英文原文，待翻譯）


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