title: Why does an SQL table have multiple primary keys?
body: <p>On my job (non programming job that takes advantage of some of my programming skills), I was asked to generate a report from our outsourced, custom made program's SQL database.</p>
  
  <p>When looking at the tables I see that many of them have multiple fields assigned as PK, why is that? What advantage does that have over a serial, numeric id?</p>