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OCA Oracle Database 11g: SQL Fundamentals I: A Real-World Certification Guide by Steve Ries

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Using database constraints

Throughout this book, we have continually noted that what makes a relational database different are the relationships formed between its tables. In Chapter 1, SQL and Relational Databases, we discussed the entity relationship diagrams that visually display these relationships. We noted that in a relational model, tables may have a one-to-one or one-to-many relationship. For instance, in our Companylink data model, each employee can have one or more addresses listed in the address table. But, what would happen if we accidentally attempted to insert address information into the address table for an employee that didn't exist in the employee table? Up to this point in what we've learned, nothing is really stopping this ...

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