O'Reilly logo

SQL: Visual QuickStart Guide, Third Edition by Chris Fehily

Stay ahead with the world's most comprehensive technology and business learning platform.

With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, tutorials, and more.

Start Free Trial

No credit card required

Chapter 3. SQL Basics

You might have noticed that I barely mentioned SQL in the preceding chapter. Remember this equation:

SQL ≠ Relational model

SQL is based on the relational model but doesn’t implement it faithfully. One departure from the model is that in SQL, primary keys are optional rather than mandatory. Consequently, tables without keys will accept duplicate rows, rendering some data inaccessible. A complete review of the many disparities is beyond the scope of this book (see the “Learning Database Design” sidebar in “Primary Keys” in Chapter 2). The upshot of these discrepancies is that DBMS users, and not the DBMS itself, are responsible for enforcing a relational structure. Another result is that the Model and SQL terms in Table 2.1 ...

With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, interactive tutorials, and more.

Start Free Trial

No credit card required