> ## Documentation Index
> Fetch the complete documentation index at: https://docs.risingwave.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Generated columns

> A generated column is a special column that is always computed from other columns. In RisingWave, you can create a generated column when creating a table or source.

To create a generated column, use the `AS <generation_expression>` clause in [CREATE TABLE](/sql/commands/sql-create-table) or [CREATE SOURCE](/sql/commands/sql-create-source) statements, for example:

```sql theme={null}
CREATE TABLE t1 (v1 int AS v2-1, v2 int, v3 int AS v2+1);
```

<Note>
  * A generation expression cannot reference another generated column.
  * The generated column is created in RisingWave and will not be accessed through the external connector. Therefore, if the external upstream system has a schema, it does not need to include the generated column within the table's schema in the external system.
</Note>

A generated column in a table is slightly different from one in a source.

* The generated column with impure expressions can not be part of the primary key.
* A generated column in a table is stored in the created table, and computed when it is inserted. This is equivalent to the `STORED` type of generated columns in PostgreSQL.
* A generated column in a source is not stored in the created source, and is computed when the source is queried. This is equivalent to the `VIRTUAL` type of generated columns in PostgreSQL.

To create a generated column as the processing time of a message, use the `proctime()` function, for example:

```sql theme={null}
CREATE TABLE t1 (v1 int, proc_time timestamptz as proctime());
```

See also [proctime()](/sql/functions/datetime#proctime).