A sequence object in SQL generates a sequence of numeric values based on specified rules, commonly used for unique number generation across sessions and tables. 1. It allows defining integers that increment or decrement by a set amount. 2. Unlike identity columns, sequences are standalone and usable across multiple tables. 3. Key properties include start value, increment, min/max bounds, and cycle option. 4. Created via the CREATE SEQUENCE statement, sequences retrieve next values using NEXT VALUE FOR. 5. Sequences are ideal for cross-table counters, pre-insert value knowledge, programmatic resets, and complex systems. 6. Identity columns suit single-table simplicity and local auto-numbering. 7. Sequences may skip numbers due to rollbacks or failures. 8. Not all databases support sequences natively, requiring workarounds.

A sequence object in SQL is a database object that generates a sequence of numeric values according to specified rules. It's commonly used to automatically generate unique numbers, such as primary key values, across multiple sessions and tables.

What Does a Sequence Object Do?

At its core, a sequence object lets you define a series of integers that can be incremented or decremented by a set amount. Unlike identity columns (which auto-increment within a single table), sequences are standalone objects — meaning they can be used across multiple tables or even in different parts of your application logic.

For example:

• You might use a sequence to generate invoice numbers that need to follow a specific format across several tables.
• Or assign ticket IDs in a support system where the same ID sequence must be referenced in more than one place.

Key properties you define when creating a sequence include:

• Start value – Where the sequence begins
• Increment – Whether it increases or decreases and by how much
• Minimum/Maximum value – Bounds for the sequence
• Cycle option – Whether it should restart after reaching min/max

How to Create and Use a Sequence in SQL

Creating a sequence is straightforward using the CREATE SEQUENCE statement. Here’s a basic example:

CREATE SEQUENCE customer_id_seq
    START WITH 100
    INCREMENT BY 1;

Once created, you can retrieve the next value from the sequence using the NEXT VALUE FOR clause:

INSERT INTO customers (id, name)
VALUES (NEXT VALUE FOR customer_id_seq, 'Jane Doe');

You’re not limited to inserts — you can also select the next value directly:

SELECT NEXT VALUE FOR customer_id_seq AS next_id;

This flexibility makes sequences useful beyond just primary keys, like generating temporary identifiers or audit logs with consistent numbering.

When Should You Use a Sequence Instead of an Identity Column?

While both sequences and identity columns generate unique numbers, there are clear use cases where one is better than the other.

Use a sequence when:

• You need the same counter across multiple tables
• You want to know the next value before inserting (e.g., for logging or validation)
• You need to reset or reseed the number generator programmatically
• You're working in environments with complex data models or distributed systems

Use an identity column when:

• The auto-numbering is strictly local to one table
• Simplicity matters and you don’t need cross-table coordination
• You don't need to pre-fetch the next value

In many cases, especially in large applications or data warehouses, sequences offer more control and flexibility.

A Few Things to Watch Out For

Sequences are powerful, but they come with a few gotchas:

• They don’t guarantee gap-free sequences. If a transaction rolls back or a session fails, some numbers may be skipped.
• In high-concurrency environments, make sure your sequence settings match your performance and scalability needs.
• Not all databases support sequences — MySQL, for example, doesn’t have them natively (you simulate similar behavior with workarounds).

Also, if you're migrating or replicating data, remember to carry over sequence definitions and current values so you don’t end up with conflicts.

That's basically how sequence objects work in SQL — they're flexible tools for generating ordered numbers outside the scope of individual tables.

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