In Hr Job Class Table, let's say we just have Job Class Code, Job Class Name, and Extract As Of Date. If we join these two tables on Extract As of Date, we will get every possible combination of these two tables. But, the information is incorrect in that we do not know anything about their actual appointments. Join tables on primary keys which are the keys which identify one row of information in a table. Or join tables with foreign keys.
Foreign keys are keys in one table which are primary keys in another table.
A good example of a foreign key is the combination of Home Dept Num and Home Dept Location Code. These two keys are in every table concerning employees. They are also the primary key in the Hr Department Table. Please call if you have questions or if you want me to use other characters in these examples.