✓ What are Sequences and why to use them
✓ Generating Unique Key values with Sequences
✓ Understanding Sequence Attributes

Sequence is a schema object that can generate unique sequential values that to integers. The sequence values are often used for Primary Keys and Unique Keys.
Sequence numbers are generated independently of tables. So, the same sequence can be used for one or for multiple tables. When a sequence number is generated, the sequence is incremented independent of the transaction is committed or rolled back.