3.5-3.9 - Creating a relational model will generate very...

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Creating a relational model will generate very many names for tables and attributes. To keep track of these names the designer is using a data dictionary. The data dictionary provides a detailed description of all tables, attributes and their characteristics. The relationships are described through the foreign keys used to link the tables. A data dictionary will contain METADATA (data about the data). For a database the data dictionary is kept in the system catalog. Additionally the system catalog will contains a description of all the other objects from the database.
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We have here a sample data dictionary. For each table in the model we have the name of the table and a description of all the attributes. In this example we have two tables: CUSTOMER and AGENT. For each attribute we have the name an explanation of the value kept, the type of the values, the usual format used to display. For instance in the table CUSTOMER, the first attribute is CUS_CODE , which will represent the customer account code. Then the type of the values is character, moreover a maximum of 5 characters is indicated. As a format is given that we will use only digits (so it will look like a number). If needed a range of values within the domain may be specified. For the CUS_CODE we have a range specified, that the first account will be 10000. Another important aspect is if an entity instance must have or not this attribute. If it is a required attribute a value must always be given. This means that the column will not contain NULL values. For instance, in the customer table: the customer code, last name and first name are required. Also is specified if the attribute is part of the primary key. In the table CUSTOMER the CUS_CODE is marked as the primary key. In the table AGENT the AGENT_CODE is marked as the primary key. All the foreign keys are also marked indicating the referenced table as well. We have a
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This note was uploaded on 01/22/2011 for the course IT 214 taught by Professor Staff during the Spring '08 term at George Mason.

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3.5-3.9 - Creating a relational model will generate very...

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