. Clusters Clusters are used in very few databases (Web site hosting)
. Clusters Clusters are used in very few databases and have been somewhat superceded by materialized views. In the past, clusters were used to pre-create physical copies of entire field level sections of heavily accessed tables, especially in SQL joins. Unlike materialized views, clusters do not allow for automatic refresh and are normally manually maintained. . Sequences and auto counters An auto counter field is a special datatype, sometimes called a non-static internal function, allowing automated generation of sequential number values (thus the term sequence ). Typically, auto counters are used for primary key surrogate key generation on insertion of new records into a table. . Partitioning and parallel processing Some databases allow physical splitting of tables into separate partitions, including parallel processing on multiple partitions and individual operations on individual partitions. One particularly efficient aspect of partitioning is the capability when querying a table to read fewer than all the partitions making up a table, perhaps even a single partition. Summary In this chapter, you learned about: . Building tables containing fields, datatypes, and simple validation . The different types of relationships between tables . Representing relations in ERDs . Defining referential integrity relationships between tables using primary and foreign keys . The types and uses of indexes . The types and uses of specialized objects such as views, materialized views, and auto counters The next chapter discusses the very heart of the relational database model by examining the process of normalization through the application of normal forms. Exercises 1. Write two CREATE TABLE commands for the tables in Option 3 of Figure 3-24. Make sure that all primary key, foreign key, and any potentially necessary unique keys are included. 2. Write CREATE INDEX commands to create all indexes on any foreign keys indicated in the CREATE TABLE command written for the previous question. 70 Chapter 3
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