Practical SQL Handbook, The: Using SQL Variants, Fourth Edition

Providing Flexibility

A corollary of the join capability is that it gives you unlimited flexibility in adding new kinds of data to your database. Joins are always possible. You can create a new table that contains data about a different subject. If the new table has an eligible field, you can link it to existing tables by joining.

That's how relational databases grow: If tables need to split, you can always reconstruct the rows by joining. If new, related information appears, you can design tables with appropriate join columns, so the new data can be linked to the existing data.

For example, our bookbiz database might need an invoices table for billing stores. Such a table could link to the sales and salesdetails tables by sonum (sales order ...