Đang chuẩn bị liên kết để tải về tài liệu:
The Language of SQL- P43

The Language of SQL- P43:Research has shown that, being pressed for time, most readers tend to skip the introduction of any book they happen to read and then proceed immediately to the first real chapter. With that fact firmly in mind, we will only cover relatively unimportant material in the introduction, such as an explanation of what you will and will not learn by reading this book. | 196 Chapter 19 Principles of Database Design Despite these provisos certain database design principles have evolved over time to guide us in our quest for an optimal design structure. It should be said from the outset that the most influential architect of relational database design is E.F. Codd who published his groundbreaking article A Relational Model of Data for Large Shared Data Banks in 1970. This article laid the foundation for what we now call the relational model and the concept of normalization. Goals of Normalization The term normalization refers to a specific process that allows designers to turn unstructured data into a properly designed set of tables and data elements. The best way to understand normalization is to illustrate what it isn t. To do this we ll start with the presentation of a poorly designed table with a number of obvious problems. The following is a table named Grades and it attempts to present information about all of the grades that students have received for the tests they ve taken. Each row represents a grade for a particular student. Test Student Date Total Points Grade TestFormat Teacher Assistant Pronoun Quiz Amy 2009-03-02 10 8 Multiple Choice Smith Collins Pronoun Quiz Jon 2009-03-02 10 6 Multiple Choice Jones Brown Solids Quiz Beth 2009-03-03 20 17 Multiple Choice Kaplan NULL China Test Karen 2009-02-04 50 45 Essay Harris Taylor China Test Alex 2009-03-04 50 38 Essay Harris Taylor Grammar Test Karen 2009-03-05 100 88 Multiple Choice Essay Smith Collins Let s first list the information that each column in this table is meant to provide. The columns are Test A description of the test or quiz given Student The student who took the test Date The date on which the test was taken TotalPoints The total number of possible points for the test Grade The number of points that the student received Goals of Normalization 197 TestFormat The format of the test either essay multiple choice or both Teacher The teacher who gave the test .