Understanding Constraints
SQL has evolved through many versions to become a very complete and powerful language. Many of these more powerful features are sophisticated database manipulation tools that provide you with data manipulation techniques such as constraints.
Relational tables and referential integrity have both been discussed several times in prior lessons. As I explained in those lessons, relational databases store data broken into multiple tables, each of which stores related data. Keys are used to create references from one table to the other (and thus the term referential integrity).
For relational database designs to work properly, you need a way to ensure that only valid data is inserted into tables. For example, if the Orders ...
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