How do you create a sequence in SQL?

You can create a sequence in SQL using the CREATE SEQUENCE statement followed by the sequence name and parameters.

In SQL, a sequence is a database object that generates a sequence of integers. It is often used to automatically generate primary key values. To create a sequence, you use the CREATE SEQUENCE statement. The basic syntax is as follows:

```
CREATE SEQUENCE sequence_name
START WITH initial_value
INCREMENT BY increment_value
MINVALUE minimum_value
MAXVALUE maximum_value
CYCLE | NOCYCLE;
```

The `sequence_name` is the name of the sequence that you want to create. The `START WITH initial_value` clause specifies the first value of the sequence. The `INCREMENT BY increment_value` clause determines the interval between sequence numbers. The `MINVALUE minimum_value` and `MAXVALUE maximum_value` clauses define the range of the sequence. The `CYCLE` keyword allows the sequence to restart from the minimum value when its maximum value is exceeded, while `NOCYCLE` prevents this.

Here's an example of creating a sequence:

```
CREATE SEQUENCE order_id_seq
START WITH 1
INCREMENT BY 1
MINVALUE 1
MAXVALUE 9999
NOCYCLE;
```

In this example, the sequence `order_id_seq` starts with 1 and will increment by 1 each time a new number is generated. The sequence will not exceed 9999 and will not cycle back to 1 when the maximum value is reached.

Once a sequence is created, you can use the `NEXTVAL` and `CURRVAL` functions to generate and retrieve sequence numbers. `NEXTVAL` increments the sequence and returns the new value, while `CURRVAL` returns the current value of the sequence without incrementing it.

Remember, sequences are database objects, so they persist across sessions and transactions. This means that even if a transaction is rolled back, the sequence number that was generated is not returned to the sequence, ensuring that each number generated is unique.