Possible problems when employing them to make surrogate keys

Question

What are sequences, and what are the possible problems when employing them to make surrogate keys?

Answer 1

A sequence is an object which makes a sequential series of exclusive numbers. Sequences are most often employed to provide values for surrogate keys. But, there are three problems with employing sequences. First, a developer can employ a defined sequence for any aim; and when a sequence is employed for purposes other than the surrogate key, some values will be absent. A second problem is that there is nothing in the schema which prevents someone from issuing an INSERT statement which adds data to the table however which does not employ the sequence. Finally, it is possible to employ the wrong sequence when putting data into a table.