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One of the most important applications for the earliest database management systems was managing operations for manufacturing companies. If an automobile manufacturer decided to produce 10,000 units of one car model and 5000 units of another model, it needed to know how many parts to order from its suppliers. To answer the question, the product (a car) had to be decomposed into hundreds of assemblies (engine, body, chassis), which were decomposed into thousands of subassemblies (valves, cylinders, spark plugs), and then into sub-subassemblies, and so on. Handling this list of parts, known as a bill of materials was a job tailor-made for computers. The bill of materials for a product has a natural hierarchical structure. To store this data, the hierarchical data model, illustrated in Figure 4-2, was developed. In this model, each record in the database represented a specific part. The records had parent/child relationships, linking each part to its subpart, and so on. To access the data in the database, a program could perform the following tasks: Find a particular part by number (such as the left door) Move down to the first child (the door handle) Move up to its parent (the body) Move sideways to the next child (the right door) Retrieving the data in a hierarchical database thus required navigating through the records: moving up, down, and sideways one record at a time.
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One of the most popular hierarchical database management systems was IBM s Information Management System (IMS), first introduced in 1968. The advantages of IMS and its hierarchical model follow. Simple structure The organization of an IMS database was easy to understand. The database hierarchy paralleled that of a company organization chart or a family tree. Parent/child organization An IMS database was excellent for representing parent/child relationships, such as A is a part of B or A is owned by B. Performance IMS stored parent/child relationships as physical pointers from one data record to another, so that movement through the database was rapid. IMS is still a widely used DBMS on IBM mainframes. Its raw performance makes it ideal for very high volume transaction-processing applications such as processing credit card transactions or booking airline reservations. Dramatic improvements in relational database performance over the last two decades have narrowed IMS s performance advantage, but the large amount of corporate data stored in IMS databases and the large number of mature applications that process that data ensure that IMS use will continue for many years to come.
Network Databases
The simple structure of a hierarchical database became a disadvantage when the data had a more complex structure. In an order-processing database, for example, a single order might participate in three different parent/child relationships, linking the order to the customer who placed it, the salesperson who took it, and the product ordered, as shown in Figure 4-3. This type of data structure simply didn t fit the strict hierarchy of IMS. To deal with applications such as order processing, a new network data model was developed. The network data model extended the hierarchical model by allowing a record to participate in multiple parent/child relationships, called sets, as shown in Figure 4-4.
FIGURE 4-3
Multiple parent/child relationships
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PART I
FIGURE 4-4
A network (CODASYL) order-processing database
In 1971, the Conference on Data Systems Languages published an official standard for network databases, called the CODASYL model. IBM never developed a network DBMS, but during the 1970s, independent software companies rushed to embrace the network model, creating products such as Cullinet s IDMS, Cincom s Total, and the Adabas DBMS that became very popular. However, IBM enhanced IMS to provide a workaround to the single-parent rule in classic hierarchical structures, calling the additional parents logical parents. The data model became known as the extended hierarchical model, and it made IMS a direct competitor with the network DBMS products. For a programmer, accessing a network database was very similar to accessing a hierarchical database. An application program could do the following: Find a specific parent record by key (such as a customer number) Move down to the first child in a particular set (the first order placed by this customer) Move sideways from one child to the next in the set (the next order placed by the same customer) Move up from a child to its parent in another set (the salesperson who took the order) Once again, the programmer had to navigate the database record by record, this time specifying which relationship to navigate as well as the direction. Network databases had several advantages: Flexibility Multiple parent/child relationships allowed a network database to represent data that did not have a simple hierarchical structure. Standardization The CODASYL standard boosted the popularity of the network model, making it easier for programmers to move between DBMS products. Performance Parent/child sets were represented by pointers to physical data records, allowing rapid navigation through these relationships.
Part I:
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