13. Self Joins and Views
The inner and outer joins of the previous two chapters dealt with various ways of combining data from multiple tables. The assumption has always been that the data exists in physical tables in a database. We’ll now turn to two techniques that will let us view data in a more virtual way. The first technique, the self join, allows the analyst to join a table to itself, referring to the same table twice, as if it were two separate tables. As such, the self join creates a virtual view of a table, allowing it to be used more than once. Second, we’ll learn about database views, which is a useful concept that enables us to create new virtual tables at will.