The Language of SQL, 3rd Edition

13

Self Joins and Views

Keywords Introduced

CREATE VIEW • ALTER VIEW • DROP VIEW

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. Now we’ll 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.

Self Joins ...