A primary key is an individual identify for each record within a table. Consider a database of HSC students within NSW. The state (ie NSW) could have two students with the same name John Smith. The database will include a field (usually called Student ID or your HSC candidate number) which is an individual number every student will have a DIFFERENT number. Therefore, one John smith may have a Candidate Number of 025739, while the other John Smith could be 025683. This way, the computer will be able to tell the difference between them.
This primary key becomes important when we link tables (using a relational database system) as the individual identifier will be use to form the links between the tables (the relationships). The student information (name address etc) may be linked to subject data (Biology, History etc) by the Primary key (in this case the Candidate number). Therefore we can set up a database which allows us to know that John Smith (025739) may be studying Biology, History and IPT, while John Smith (025683) is studying Japanese, Physics and Chemistry.
