In relational database management, to match one table (file) against another based on some condition creating a third table with data from the matching tables. For example, a customer table can be joined with an order table creating a table for all customers who purchased a particular product.
The default type of join is known as an "inner" join. It produces a resulting record if there is a matching condition. For example, matching shipments with receipts would produce only those shipments that have been received. On the other hand, an "outer" join using that example would create a record for every shipment whether or not it was received. The data for received items would be attached to the shipments, and empty, or null, fields would be attached to shipments without receipts.
A Simple Join
This example matches the sales table against the product table based on product number to derive the product description.