3: Database Design in .NET framework

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3: Database Design
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Referential integrity ensures that created relationships between tables are maintained It is maintained by creating relationships between tables with PRIMARY and FOREIGN KEYs and among other checks, it ensures that data in a referenced table aren t deleted In the following sections, we ll first cover the primary and FOREIGN KEYs and then some other types of constraints
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PRIMARY KEYs
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Each row within a table must be unique This is referred to as entity integrity and is accomplished with the use of the PRIMARY KEY (PK) constraint A PRIMARY KEY is a field or group of fields that uniquely identifies a row Tables can have only one PK constraint, and it can t accept null values Consider our Employees table from 2 We can t afford to have two employees with all of their data the same Even if you were to have two employees named John Smith, a PK such as EmployeeID would be used to enforce uniqueness between the two PRIMARY KEYs can be created using either numeric or character data types, though they are often defined as an integer data type with the IDENTITY property Each time we add a new employee, the IDENTITY property will automatically create a new unique number for that employee Identity needs two parameters, as in IDENTITY(100,1) The first number says what number to start with (referred to as the seed), and the next number says how much to increment for the next employee With IDENTITY (100,1), the seed is 100 and the increment is 1; the first EmployeeID would be 100, and the next EmployeeID would be 101 It s also possible to have a composite PK What this means is that instead of using just a single column as the PK, we use two or more columns This is frequently done in a junction table creating a many-to-many relationship between two tables We ll discuss junction tables and many-to-many relationships later in this chapter
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This chapter covers many scripts, which are available on the accompanying CD Updated scripts (if there is any need for updates) can be found at MCITPSuccess com Once you ve downloaded them, you can easily cut and paste them into SSMS, but I strongly encourage you to type them in You ll make mistakes, yes, but as you do, you ll learn As you correct your typos, you ll learn more So why are the scripts available Sometimes, especially after typing for a while, the smallest typos elude us Instead of stopping in frustration, you can always cut and paste in the scripts, and if desired, compare yours side by side with the book scripts
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A simple DDL script to create an Employee table with the EmployeeID as the PRIMARY KEY is shown in the following code Notice the PK is using the Identity property
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USE Master CREATE DATABASE 3; GO
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MCITP SQL Server 2005 Database Developer All-in-One Exam Guide
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USE 3; GO CREATE TABLE dboEmployee ( EmployeeID int IDENTITY(100,1) NOT NULL, LastName varchar(35) NULL, FirstName varchar(35) NULL, CONSTRAINT [PK_Employee_EmployeeID] PRIMARY KEY CLUSTERED ( EmployeeID ) )
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Figure 3-1 shows what the table would look like in the SSMS graphical user interface (GUI) The key icon next to EmployeeID gives us a graphical representation of the column that is the PRIMARY KEY
Figure 3-1 The Employee table in SSMS
EXAM TIP While it is possible to create tables in the SSMS GUI, what you ll often see on test questions is a script showing the structure of a table You should be able to look at the script and be able to picture what the table looks like This becomes a lot easier after you ve typed in a few
FOREIGN KEYs
A FOREIGN KEY (FK) is used to establish a relationship between two tables The FK in one table will typically refer to the PK in the other table, (but could also refer to a UNIQUE constraint instead of a PK) This relationship provides referential integrity between the two tables While the primary purpose of the FOREIGN KEY constraint is to link the two tables together so that a row in one table is linked to a row in another table, it does more It controls changes to data in both the PRIMARY KEY table and the FOREIGN KEY table This will become clearer as we explore the FOREIGN KEY more Let s consider our Employee table Notice that there are no address entries We could have put the address, city, state, and zip columns in the Employee table, but imagine that our database has other tables for Customers, Suppliers, and Contractors Each of these tables needs address information The address information we put in one table, we would have to repeat in all the tables To reduce this redundancy, we create a separate table named Address and then relate the Employee and the Address table with a PK-to-FK relationship The Address table has a PK constraint named AddressID We add an FK constraint named AddressID to the Employees table that references the AddressID field in the Address table (We could also relate the Address table to the Customers, Suppliers, and Contractors tables) This is done in the AdventureWorks database
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