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Why would we create a sequence even if there is a primary key?
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A sequence is a database object that allows the automatic generation of values, such as check numbers. Sequences are ideally suited to the task of generating unique key values. Applications can use sequences to avoid possible concurrency and performance problems resulting from column values used to track numbers.
A sequence is a database object which allows users to generate unique integer values. The sequence is incremented every time a sequence number is generated. The incrementation occurs even if the transaction rolls back, which may result in gaps between numbers.
These databases collect all publicly available DNA, RNA and protein sequence data and make it available for free.
The Protein database is a collection of sequences from several sources, including translations from annotated coding regions in GenBank, RefSeq and TPA, as well as records from SwissProt, PIR, PRF, and PDB. Protein sequences are the fundamental determinants of biological structure and function.
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.
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