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CHAPTER 1 RELATION AL DATABA SE OV ERVIEW
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the domain might be integer values greater than 0. All database systems have built-in domains or types such as text, integer, or date that can be chosen for each of the fields in a table. Systems vary as to whether users can define their own more highly specified domains that they can use across different tables; however, all good database systems allow the designer to specify constraints on a particular attribute in a table. For example, in a particular table we might specify that a birth date is a date in the past, that the value for a gender field must be M or F , or that a student s exam mark is between 0 and 100. The idea of domains becomes important for queries when we need to compare values of columns in different tables. When I refer to a database table in this book, I mean a set of rows with a nominated primary key to ensure every row is different and where every column has a domain of allowed values. Listing 1-1 shows the SQL code for creating the Member table with the attribute names and domains specified. In SQL, the keyword INT means an integer or nonfractional number, and CHAR(n) means a string of characters n long. The code also specifies that MemberID will be the primary key. (Listing 1-1 doesn t create the relationship with the Team table yet.) The code is fairly self-explanatory.
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Listing 1-1. SQL to Create the Member Table CREATE TABLE Member ( MemberID INT PRIMARY KEY, LastName CHAR(20), FirstName CHAR(20), Phone CHAR(20), Handicap INT, JoinDate DATETIME, Gender CHAR(1))
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Inserting and Updating Rows in a Table
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The emphasis in this book is on getting accurate information out of a database, but the data has to get in somehow. Most database application developers will provide user-friendly interfaces for inserting data into the various tables. Often a form is presented to the user for entering data that may end up in several tables. Figure 1-4 shows a simple Microsoft Access form that allows a user to enter and amend data in the Member table. It is also possible to construct web forms or mechanical readers (such as the bar-code readers at supermarkets) that collect data and insert it into a database. Behind all the different interfaces for updating data, SQL update queries are generated. I will show you three types of queries for inserting or changing data just so you get an idea of what they look like. I think you will find them quite easy to understand.
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CHAPTER 1 RELA TION AL DA TA BAS E OVERVIEW
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Figure 1-4. A form allowing entry and updating of data in the Member table
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Listing 1-2 shows the SQL to enter one complete row in our Member table. The data items are in the same order as specified when the table was created (Listing 1-1). Note that the date and string values need to be enclosed in single quotes.
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Listing 1-2. Inserting a Complete Row into the Member Table INSERT INTO Member VALUES (118, 'McKenzie', 'Melissa', '963270', 30, '05/10/1999', 'F')
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If many of the data items are empty, we can specify which attributes or fields will have values. If we had only the ID and last name of a member, we could insert just those two values as in Listing 1-3. Remember that we always have to provide a value for the primary key field.
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Listing 1-3. Inserting a Row into the Member Table When Only Some Attributes Have Values INSERT INTO Member (MemberID, LastName) VALUES (258, 'Olson')
We can also alter records that are already in the database with an update query. Listing 1-4 shows a simple example. The query needs to identify which records are to be changed (the WHERE clause in Listing 1-4) and then specify the field or fields to be updated (the SET clause).
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