1. Record-based logical models:

• Also describe data at the conceptual and view levels.
• Unlike object-oriented models, are used to Specify overall logical structure of the database, and Provide a higher-level description of the implementation.
• Named so because the database is structured in xed-format records of several types.
• Each record type de nes a xed number of elds, or attributes.
• Each eld is usually of a xed length (this simpli es the implementation).
• Record-based models do not include a mechanism for direct representation of code in the database.
• Separate languages associated with the model are used to express database queries and updates.
• The three most widely-accepted models are the relational, network, and hierarchical.
• This course will concentrate on the relational model.
• The network and hierarchical models are covered in appendices in the text.

The Relational Model

• Data and relationships are represented by a collection of tables.
• Each table has a number of columns with unique names, e.g. customer, account
• Figure 1.3 shows a sample relational database.

Figure 1.3: A sample relational database.

Figure 1.4: A sample network database

The Network Model

• Data are represented by collections of records.
• Relationships among data are represented by links.
• Organization is that of an arbitrary graph.
• Figure 1.4 shows a sample network database that is the equivalent of the relational database of Figure 1.3.

The Hierarchical Model

• Similar to the network model.
• Organization of the records is as a collection of trees, rather than arbitrary graphs.
• Figure 1.5 shows a sample hierarchical database that is the equivalent of the relational database of Figure 1.3.

Figure 1.5: A sample hierarchical database

The relational model does not use pointers or links, but relates records by the values they contain. This allows a formal mathematical foundation to be defined.