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The Relational Model
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This concept of interchangeability can be extended further to table expressions queries that return relations inside outer queries. You probably already know about derived tables and common table expressions; you ll learn how to use them ef ciently in 6, Subqueries, Table Expressions, and Ranking Functions.
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Naming Conventions
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Naming conventions help you make more intuitive designs and write clearer code. Your choice of convention is not as important as choosing a convention and using it consistently; I do not want to force a particular one on you. Conventions are a matter of history, taste, system limitations, and so on. Database designers tend to get really passionate about naming conventions. I like the predicate-and-propositions approach to the meaning of relations. For example, I am repeating the proposition I already mentioned: The employee with ID number 17 is named Fernando, works in department D1, and was hired on July 19th, 2003. I suggest that you should always be able to re-create the predicates and the propositions. A tuple that represents this proposition is written in a relation with values only, like (17, Fernando, D1, 2003-07-19). It is easy to recreate this proposition if its predicate, i.e. table structure, has meaningful names for table itself and for columns, like Employees(EmployeeId, EmployeeName, DepartmentId, HireDate). However, if the table and the columns would be named Table1(column1, column2, column3, column4). In short, you should be able to read your database. This makes it simpler to determine whether your database serves your business problem well and whether your data is in accordance with business rules. It also makes it much simpler to familiarize a new programmer with the database design and makes the task of data interchange with other systems easier. The only naming convention I really do not like for a relational database is the one called Hungarian notation, in which you use pre xes to denote object types. Hungarian notation uses names like tblEmployees for an employee table and vwCustomerOrders for a customer orders view; such names contradict the principle of interchangeability, which is one of the most important principles in the relational model.
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The Relational Model: A Quick Summary
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The relational model is background independent, which means it does not depend on any speci c presumption. I will return to this fact multiple times. To begin, let me state explicitly that the relational model is not type dependent. There are no prescribed relational types, and there are no beyond relational types. The relational model allows any type at all. In fact, it is completely valid to de ne a relation with a single attribute of a quite complex type; this would be a typed relation. However, system-supplied types are usually easier to use because database developers already know how to use them and, of course, don t have to develop them from scratch.
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Inside Microsoft SQL Server 2008: T-SQL Querying
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To summarize, the relational model consists of the following components:
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An open-ended collection of scalar types A way to de ne types in other words, a type generator A way to de ne relation types in other words, a relation type generator A way to generate relational variables and assign values (sets) to them Relational algebra: an open-ended collection of relational operators
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Tables represent relations, and all information in a relational database is stored in tables. A relation represents an entity from the real world. In addition, tuples of a relation represent propositions, and a relation header represents a predicate. The relational model is not dependent on naming conventions, either. Again, it is background independent. This means it is your responsibility to use a naming convention descriptive enough to make it possible to re-create predicates and propositions from your database.
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