In this chapter, we've brought the key concepts of an RDBMS together with the power of SQL. Using Data Definition Language (DDL), we've learned to create tables with different datatypes that specify the kinds of data we wish to store. We've enforced business rules with different types of database constraints, including primary and foreign keys, unique keys, not nulls, and check constraints. Finally, we brought this knowledge into the real world by applying constraints to our
Companylink tables, as well as creating new ones.