What is a sequence (Database)? When would we need it?

The primary key is a column in a table.

The primary key needs a unique value, which needs to come from somewhere.

The sequence is a feature by some database products which just creates unique values. It just increments a value and returns it. The special thing about it is: there is no transaction isolation, so several transactions can not get the same value, the incrementation is also not rolled back. Without a database sequence it is very hard to generate unique incrementing numbers.

Other database products support columns that are automatically initialized with a incrementing number.

There are other means to create unique values for the primary keys, for instance Guids.

Sequence will allow you to populate primary key with a unique, serialized number.

It's different from a serial or auto_incremement primary key in the sense that:

• It is an actual database object (you need to create it):

  sql> create sequence NAME_OF_YOUR_SEQUENCE;

• You could assign independent permissions to it, to different database users:

  sql> grant select on NAME_OF_YOUR_SEQUENCE to NAME_OF_YOUR_USER;

• You can use to have a unique number that is different among several tables (not just one). Say you have four tables with numeric primary keys, and you want unique numbers among those four tables. You could use a sequence for that, without having to worry about implementing locking mechanisms to do it 'by hand'.

• You can change its number to any value you want with alter sequence

• You can cycle through its numbers

  sql> create sequence NAME_OF_YOUR_SEQUENCE maxvalue 1500 cycle;