Database Systems: The Complete Book- P3: Database Systems and Database Design and Application courses offered at the junior, senior and graduate levels in Computer Science departments. Written by well-known computer scientists, this introduction to database systems offers a comprehensive approach, focusing on database design, database use, and implementation of database applications and database management systems | 176 CHAPTER 4. OTHER DATA MODELS data structures that support efficient answering of queries as we shall discuss beginning in Chapter 13. Yet the flexibility of semistructured data has made it important in two applications. We shall discuss its use in documents in Section but here we shall consider its use as a tool for information integration. As databases have proliferated it has become a common requirement that data in two or more of them be accessible as if they were one database. For instance companies may merge each has its own personnel database its own database of sales inventory product designs and perhaps many other matters. If corresponding databases had the same schemas then combining them would be simple for instance we could take the union of the tuples in two relations that had the same schema and played the same roles in the the two databases. However life is rarely that simple. Independently developed databases are unlikely to share a schema even if they talk about the same things such as personnel. For instance one employee database may record spouse-name another not. One may have a way to represent several addresses phones or emails for an employee another database may allow only one of each. One database might be relational another object-oriented. To make matters more complex databases tend over time to be used in so many different applications that it is impossible to shut them down and copy or translate their data into another database even if we could figure out an efficient way to transform the data from one schema to another. This situation is often referred to as the legacy-database problem- once a database has been in existence for a while it becomes impossible to disentangle it from the applications that grow up around it so the database can never be decommissioned. A possible solution to the legacy-database problem is suggested in Fig. . We show two legacy databases with an interface there could be many legacy systems involved.
