The join seems to be unique to database systems but we developers use them in other processing, as you will see. Joins are a variation on sets which is how database queries work. Name,SaleFile.saleamount From PersFile join persFile. Empid Where PersFile. It occurred to me that one way of understanding joins would be to fall back to my third generation language experience and try to duplicate the actions of a join.
Joins show how tables are related.
Or, even better, joins relate tables. Any two given tables have a relationship, even if it isn't readily apparent. We usually talk about a common field. That is, there is a field in each table which contains the same value. This field is used in matching records.