Oracle - SQL - Creating Sequences

Summary

This Oracle SQL tutorial introduces sequences, which automatically generate unique numeric values, often used for primary key columns. The video covers how to create a sequence, set its parameters (like start value, increment, max value, and caching), and use it in SQL inserts. It also explains how to query sequence details and utilize `CURRVAL` and `NEXTVAL` to retrieve the current or next sequence value. The tutorial emphasizes practical implementation, showing step-by-step instructions for linking sequences to tables and ensuring unique identifiers for records.

Takeaways

• 😀 Sequences in Oracle SQL generate unique numeric values, typically used for primary key columns in tables.
• 😀 A sequence ensures that the generated values are unique for each insert operation, providing automatic value generation.
• 😀 Sequences can be shared across multiple tables, meaning a single sequence can generate values for different tables.
• 😀 The `CREATE SEQUENCE` statement allows users to define attributes such as start value, increment value, maximum value, and cache size.
• 😀 The `START WITH` clause specifies the initial value for the sequence, and the `INCREMENT BY` clause determines the step size between generated values.
• 😀 You can set a `MAXVALUE` for the sequence to limit how high the generated values can go; otherwise, it defaults to no limit.
• 😀 The `CYCLE` option allows the sequence to restart once it reaches the maximum value, while `NO CYCLE` prevents this behavior (default).
• 😀 The `CACHE` option helps speed up sequence generation by preloading a specified number of sequence values into memory.
• 😀 You can query sequence properties using the `USER_SEQUENCES` view to check attributes such as the `START WITH`, `INCREMENT BY`, and `MAXVALUE`.
• 😀 To use a sequence, the `NEXTVAL` function can be used to retrieve the next value in the sequence during insert operations, ensuring unique values for columns.
• 😀 Sequences are especially useful for auto-generating primary key values in a table and ensuring data consistency and uniqueness.

Q & A

What is the purpose of a sequence in Oracle SQL?

A sequence in Oracle SQL is used to automatically generate unique numeric values, typically for use as primary keys in a table.

How do you create a sequence in Oracle SQL?

You create a sequence using the `CREATE SEQUENCE` statement, followed by the sequence name and optional parameters such as `START WITH`, `INCREMENT BY`, `MAXVALUE`, `CYCLE`, and `CACHE`.

What is the default behavior when the `MAXVALUE` is not specified in a sequence?

If `MAXVALUE` is not specified, the sequence will have no upper limit by default.

What does the `CYCLE` option in a sequence do?

The `CYCLE` option allows the sequence to restart from the beginning once it reaches its maximum value. If `NOCYCLE` is chosen (default), the sequence will not restart after reaching the maximum value.

What is the `CACHE` option in a sequence used for?

The `CACHE` option is used to store a set number of sequence values in memory, improving performance by reducing disk access. If not specified, the default is 20 cached values.

What is the difference between `NEXTVAL` and `CURRVAL` in sequence usage?

`NEXTVAL` retrieves the next value from the sequence and increments it, while `CURRVAL` retrieves the current value without changing it.

How do you insert a value from a sequence into a table in Oracle SQL?

You use the sequence's `NEXTVAL` in an `INSERT` statement to generate a unique value, such as: `INSERT INTO table_name (column1) VALUES (sequence_name.NEXTVAL);`.

Can a sequence be shared across multiple tables?

Yes, a sequence can be shared across multiple tables, allowing it to generate unique values for primary key columns in different tables.

How can you check the current values of a sequence in Oracle SQL?

You can query the `USER_SEQUENCES` view to retrieve information about a sequence, such as its increment value, maximum value, and whether it cycles or caches values.

What happens if you try to insert more values than the `MAXVALUE` of a sequence?

If the sequence reaches its `MAXVALUE` and `CYCLE` is not enabled, it will stop generating new values. If `CYCLE` is enabled, it will start over from the minimum value.