Objectives: Introduction Over View of System Analysis and Design
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- 7.7.3 Relational Structuring
| 7.7.2 Network Structuring A network structure allows 1:1, M, or MM relationships among entities. For example, an auto parts shop may have dealings with more than one manufacturer parent. Spare parts may come from two companies, so they are owned by both entities-a structure that can best be supported by a network. Now consider the manufacturer and the auto parts shops it deals with. If the manufacturer sold spare parts to only one shop (say, anew car dealer, then there is a 1:1 relationship. If it supplied to many other dealers, then there is a M relationship. The 1:1 and M relationships can be supplied by a hierarchy. When auto parts dealers are supplied by many manufacturers, however, there is an MM relationship, which is a network structure. A network structure reflects the real world, although a program structure can become complex. The solution is to separate the network into several hierarchies with duplicates. This simplifies the relationship to no more complex than MA hierarchy, then becomes a subview of the network structure. 7.7.3 Relational Structuring In relational structuring, all data and relationships are represented in a flat, two- dimensional table called a relation. A relation is equivalent to a file, where each line represents a record. For example, a relation that describes the entity EMPLOYEE by social security number, name and years with the firm. All the entries in each field are of the same kind. Furthermore, each field has a unique name. Finally, no two rows in the table are identical. A row is referred to as a tuple. A relational DBMS has several features 1. It allows the user to update (add, modify, or delete) the table’s contents. Any position can be changed. 2. It provides inquiry capabilities against a label. Using our example, an inquiry might be How many years has Boynton been with the firm The response is “6.” 3. Two or more tables can be merged to form one relation. Unlike hierarchical or network structuring where all relationships are predefined, a relational DBMS develops new relations on user commands. 4. A relational structure is simpler to construct than a hierarchical or a network structure. It maybe inefficient, though, since a relational DBMS responds to queries by an exhaustive review of the relations involved. Download 0.94 Mb. Share with your friends:
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