SQL Joins

SQL JOIN is used to combine two or more tables based on primary key and foreign keys.

We use Join in conjunction with the table we normalize during database design. Consider a Department table we have in our previous example. We create a Department table that serves as our master file to be called from other table.

SQL JOIN can be very time consuming on large table, e.g. one with millions of records, since it has to match each row of one table, and check it with each row of other tables mentioned in the JOIN keyword.