Một khách hàng có thể có một hoặc nhiều tài khoản. Tuy nhiên, mỗi tài khoản thuộc về khách hàng một cách chính xác. Tương tự như vậy, một hoặc nhiều khách hàng có thể được trong một hộ gia đình. | 482 Chapter 15 WHAT IS A RELATIONAL DATABASE continued One account has multiple transactions but each transaction is associated with exactly one account. A single transaction occurs at exactly one vendor. But each vendor may have multiple transactions. An E-R diagram can be used to show the tables and fields in a relational database. Each box shows a single table and its columns. The lines between them show relationships such as 1-many 1-1 and many-to-many. Because each table corresponds to an entity this is called a physical design. Sometimes the physical design of a database is very complicated. For instance the TRANSACTION TABLE might actually be split into a separate table for each month of transactions. In this case the above E-R diagram is still useful it represents the logical structure of the data as business users would understand it. An entity relationship diagram describes the layout of data for a simple credit card database. With respect to data mining relational databases and SQL have some limitations. First they provide little support for time series. This makes it hard to figure out from transaction data such things as the second product purchased the last three promos a customer responded to or the ordering of events these can require very complicated SQL. Another problem is that two operations often eliminate fields inadvertently. When a field contains a missing value NULL then it automatically fails any comparison even not equals . Team-Fiy Data Warehousing OLAP and Data Mining 483 Also the default join operation called an inner join eliminates rows that do not match which means that customers may inadvertently be left out of a data pull. The set of operations in SQL is not particularly rich especially for text fields and dates. The result is that every database vendor extends standard SQL to include slightly different sets of functionality. Database schema can also illuminate unusual findings in the data. For instance we once worked with a file .
