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SQL: Visual QuickStart Guide by Chris Fehily

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Chapter 3. SQL Basics

You may 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 isn’t a faithful implementation of it. One departure from the model is that in SQL, primary keys are optional rather than mandatory. Consequently, tables without keys may have duplicate rows, rendering some data inaccessible. A complete review of the disparities is beyond the scope of this book (if you want to learn more, search the Web for SQL articles by E.F. Codd, Chris Date, or Fabian Pascal). The upshot of these discrepancies is that DBMS users, and not the DBMS itself, bear the responsibility for enforcing a relational structure. Another result is that the ...

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