The Primary Key

Relational Databases

A relational database is a system in which information flows from one database object to another. For example, on a bank database, you can use one object to create accounts for customers and use another object to process transactions that the owners of bank accounts need. The reason is that the same customer may need to perform various transactions, regularly. Instead of creating a new account every time the customer wants to perform a new transaction, you can use one account as a reference and bring up this account whenever the customer wants to deposit or withdraw money.

To apply the rules of relational databases, you create some types of relationships among the objects of the database.

To create a primary key in SQL Server Management Studio, create a column and specify its data type:

• Then, on the toolbar, click the Set Primary Key button
• You can also right-click a column and click Set Primary Key

Here is an example:

To create a primary column using SQL, the primary thing to do is, on the right side of the column definition, type PRIMARY KEY. Here is an example:

CREATE TABLE Persons
(
    PersonID int identity(1,1) PRIMARY KEY NOT NULL,
    FirstName varchar(20),
    LastName varchar(20) NOT NULL
);

In the SQL, you can give a specific name to a primary key. To do this, you can first create the column. Then, somewhere before the closing parenthesis of the table, specify the primary key column using the following formula:

CONSTRAINT PrimaryKeyName PRIMARY KEY(ColumnName)

In this formula, the CONSTRAINT keyword and the PRIMARY KEY (case-insensitive) expression are required. In the PrimaryKeyName placeholder, enter the name you want to give to the primary key. In the parentheses of the PRIMARY KEY expression, enter the name of the column that will be used as the primary key. Here is an example:

CREATE TABLE Persons
(
    PersonID int identity(1,1) NOT NULL,
    FirstName varchar(20),
    LastName varchar(20) NOT NULL,
    CONSTRAINT PrimKeyPeople PRIMARY KEY(PersonID)
);

By convention or tradition, the name of the primary starts with PK_ followed by the name of the table. Here is an example:

USE Exercise2;
GO

CREATE TABLE Persons
(
    PersonID int identity(1,1) NOT NULL,
    FirstName varchar(20),
    LastName varchar(20) NOT NULL,
    CONSTRAINT PK_Persons PRIMARY KEY(PersonID)
);
GO

Continuing with our bank database, imagine a customer comes to the bank to deposit money. We already established that it would be redundant to create a new account every time the customer comes to perform a transaction. Instead, you would get the customer's information from his or her account, provide that information to the table used to process transactions. As we described earlier, the account table should be able to provide its data to the other tables that would need that data. To make this flow of information possible from one table to another, you must create a relationship between them.

To make it possible for a table B to receive data from a table A, the table B must have a column that represents the table A. This columns acts as an "ambassador" or a link. As a pseudo-ambassador, the column in the table B almost doesn't belong to that table: it primarily allows both tables to communicate. For this reason, the column in the table B is called a foreign key.

A foreign key is a column on a table whose data is coming from another table.

To create a foreign key in the Table Design window, in the table that will receive the key, simply create a column with the following rules:

• The column should have the same name as the primary column of the table it represents (but this is not a requirement)
• The column must (this is required) have the same data type as the primary column of the table it represents

Here is an example of a column named GenderID that is a foreign key:

Obviously in order to have information flowing from one table to another, the table that holds the primary information must be created. You can create it before or after creating the other table, as long as you haven't established any link between both tables, it doesn't matter what sequence you use to create them.

The table that contains a primary key and that holds the information that another table would use is called the primary table or the parent table. The table that will receive the information from the other table is called the foreign table or the child table.

To create a foreign key in a table:

1. From the Object Explorer, open the child table in Design View
2. Right-click anywhere in the table and click Relationships...
    
   
3. In the Foreign Key Relationships dialog box, click Add
4. A default name would be suggested to you. You can accept or change it. To change the name of the foreign key, in the right side, expand Identity and edit the string in the (Name) field:
    
   
5. If necessary, in the same way, you can create other foreign keys by clicking Add. To delete an existing foreign key, first select it under Selected Relationships and click Delete.
   Once you are ready, click Close

You can also create a foreign key in the SQL. The basic formula to use is:

FOREIGN KEY REFERENCES ParentTableName(ForeignKeyCcolumn) 

The FOREIGN KEY expression and the REFERENCES keyword are required. In the ParentTableName placeholder, enter the name of the primary table that holds the information that will be accessed in the current table. In the parentheses of ParentTableName, enter the name of the primary column of the parent table. Here is an example:

CREATE TABLE Persons
(
    PersonID int identity(1,1) PRIMARY KEY NOT NULL,
    FirstName varchar(20),
    LastName varchar(20) NOT NULL,
    GenderID int NULL FOREIGN KEY REFERENCES Genders(GenderID)
);

Notice that the foreign key doesn't have an object name as we saw for the primary key. If you don't specify a name for the foreign key, the SQL interpreter would automatically create a default name for you. Otherwise, to create a name, after creating the column, enter the CONSTRAINT keyword followed by the desired name and continue the rest as we saw above. Her is an example:

CREATE TABLE Persons
(
    PersonID int identity(1,1) PRIMARY KEY NOT NULL,
    FirstName varchar(20),
    LastName varchar(20) NOT NULL,
    GenderID int NULL CONSTRAINT FKGenders
                       FOREIGN KEY REFERENCES Genders(GenderID)
);

As mentioned already, a relational database is one in which information flows from one table to another. To prepare the tables for this, you create primary and foreign keys, which we have done so far. Once the tables are ready, you can link them, which is referred to as creating a relationship between two tables.

If you didn't create a foreign key with SQL code, you can create it when establishing a relationship between two tables.

To create a relationship between two tables

1. Open the child table in the design view
2. Right-click (anywhere in) the table and click Relationships...
   If the (necessary) foreign key doesn't exist, click Add and specify its name under Identity) in the right side.
3. Under Selected Relationships, click the foreign key that will hold the relationship
4. In the right side, expand Tables And Columns Specification
5. Click its ellipsis button
6. In the Primary Key Table combo box, select the parent table that holds the primary data
7. Under the parent table, click and select its primary key column
8. Under Foreign Key Table, make sure the name of the current table is set.
   Under the name of the child table, click and select the name of the foreign key column. Here is an example:
    
   
9. Click OK.
   When a relationship has been created, it would show in the Tables And Column Specification section:
    
   
10. In the same way, you can create other relationships by clicking Add and configuring the link.
    Once you have finished, click Close

1. To complete our database with its tables, their primary keys, their foreign keys, and some data in the tables, change the code in the query window as follows:
    

   -- ============================================= -- Database: YugoNationalBank -- Author: FunctionX -- Date Created: Monday 09 April 2007 -- ============================================= USE master GO -- Drop the database if it already exists IF EXISTS ( SELECT name FROM sys.databases WHERE name = N'YugoNationalBank' ) DROP DATABASE YugoNationalBank GO CREATE DATABASE YugoNationalBank GO -- ========================================= -- Database: YugoNationalBank -- Table: Locations -- ========================================= USE YugoNationalBank GO IF OBJECT_ID('dbo.Locations', 'U') IS NOT NULL DROP TABLE dbo.Locations GO CREATE TABLE Locations ( LocationID int Identity(1,1) NOT NULL, LocationCode varchar(10) NOT NULL, Address varchar(120), City varchar(50), State varchar(50), Notes text NULL, CONSTRAINT PK_Locations PRIMARY KEY (LocationID) ); GO -- ========================================= -- Database: YugoNationalBank -- Table: AccountTypes -- ========================================= USE YugoNationalBank GO IF OBJECT_ID('dbo.AccountTypes', 'U') IS NOT NULL DROP TABLE dbo.AccountTypes GO CREATE TABLE AccountTypes ( AccountTypeID int Identity(1,1) NOT NULL, AccountType varchar(40) NOT NULL, Notes text NULL, CONSTRAINT PK_AccountTypes PRIMARY KEY (AccountTypeID) ); GO -- ========================================= -- Database: YugoNationalBank -- Table: Employees -- ========================================= IF OBJECT_ID('dbo.Employees', 'U') IS NOT NULL DROP TABLE dbo.Employees GO CREATE TABLE Employees ( EmployeeID int identity(1,1) NOT NULL, EmployeeNumber char(6), FirstName varchar(32), LastName varchar(32) NOT NULL, Title varchar(50), CanCreateNewAccount bit, HourlySalary smallmoney, EmailAddress varchar(100), Username varchar(20), Password varchar(20), Notes text, CONSTRAINT PK_Employees PRIMARY KEY (EmployeeID) ); GO -- ========================================= -- Database: YugoNationalBank -- Table: Customers -- ========================================= IF OBJECT_ID('dbo.Customers', 'U') IS NOT NULL DROP TABLE dbo.Customers GO CREATE TABLE Customers ( CustomerID int Identity(1,1) NOT NULL, DateCreated datetime, AccountNumber varchar(20), AccountTypeID int Constraint FK_TypeOfAccount References AccountTypes(AccountTypeID), CustomerName varchar(50) NOT NULL, DateUpdated smallDateTime, Notes text, CONSTRAINT PK_Customers PRIMARY KEY (CustomerID) ); GO -- ========================================= -- Database: YugoNationalBank -- Table: ChargeReasons -- ========================================= IF OBJECT_ID('dbo.ChargeReasons', 'U') IS NOT NULL DROP TABLE dbo.ChargeReasons GO -- ========================================= -- Database: YugoNationalBank -- Table: Deposits -- ========================================= IF OBJECT_ID('dbo.Deposits', 'U') IS NOT NULL DROP TABLE dbo.Deposits GO CREATE TABLE Deposits ( DepositID int identity(1, 1) NOT NULL, LocationID int Constraint FK_DepositLocation References Locations(LocationID) NOT NULL, EmployeeID int Constraint FK_Clerk References Employees(EmployeeID), CustomerID int Constraint FK_Depositor References Customers(CustomerID) NOT NULL, DepositDate smalldatetime NOT NULL, DepositAmount smallmoney NOT NULL, Notes text, CONSTRAINT PK_Deposits PRIMARY KEY (DepositID) ); GO -- ========================================= -- Database: YugoNationalBank -- Table: Withdrawals -- ========================================= IF OBJECT_ID('dbo.Withdrawals', 'U') IS NOT NULL DROP TABLE dbo.Withdrawals GO CREATE TABLE Withdrawals ( WithdrawalID int identity(1, 1) NOT NULL, LocationID int Constraint FK_WithdrawlLocation References Locations(LocationID) NOT NULL, EmployeeID int Constraint FK_ProcessedBy References Employees(EmployeeID), CustomerID int Constraint FK_CustomerAccount References Customers(CustomerID) NOT NULL, WithdrawalDate smalldatetime NOT NULL, WithdrawalAmount smallmoney NOT NULL, WithdrawalSuccessful bit NOT NULL, Notes text, CONSTRAINT PK_Withdrawas PRIMARY KEY (WithdrawalID) ); -- ========================================= -- Database: YugoNationalBank -- Table: CheckCashing -- ========================================= IF OBJECT_ID('dbo.CheckCashing', 'U') IS NOT NULL DROP TABLE dbo.CheckCashing GO CREATE TABLE CheckCashing ( CheckCashingID int identity(1, 1) NOT NULL, LocationID int Constraint FK_BranchLocations References Locations(LocationID) NOT NULL, EmployeeID int Constraint FK_Employees References Employees(EmployeeID), CustomerID int Constraint FK_Customers References Customers(CustomerID) NOT NULL, CheckCashingDate smalldatetime NOT NULL, CheckCashingAmount smallmoney NOT NULL, CheckCashingSuccessful bit NOT NULL, Notes text, CONSTRAINT PK_CheckCashing PRIMARY KEY(CheckCashingID) ); GO

2. Press F5 to execute the statement

A diagram is a window that visually displays the relationships among tables of a database. To create a diagram:

1. In the Object Explorer, in the database node, you can click Database Diagrams
2. A dialog box will inform you that this database doesn't have a diagram. Read the message and click Yes
3. Right-click Database Diagrams and click New Database Diagram
4. In the Add Table dialog box, click each table and click the Add.
   Alternatively, you can double-click a table to add it
5. In the Add Table dialog box, you can click Close.
   On the toolbar, you can click the Zoom button and select a larger or smaller value.
   To move a table, you can drag its title bar. Here is an example:
    
   
6. To establish a relationship, you can click the gray box on the left of any column from the parent table and drop it on any column in the child table. A better way is to click gray box of the primary key column from the parent table, drag that box then drop it on the foreign key column of the child table. Here is an example:
    
   
7. A Tables and Columns dialog box would come up. It would display the column that was dragged and the column on which you dropped.
   If you had selected just any column, it would show but it may not be the one you wanted to drag; that is, it may not be the actual column that is supposed to manage the relationship.
   Regardless, under Primary Key Table, you should select the parent table
8. Under the parent table, select its primary column
9. Under Foreign Table, select the foreign key column. Here is an example:
    
   
10. Once you are ready, click OK. A link would be created between the tables
     
    
11. In the same way, you can create other relationships.
    When you have finished, you can save and close the database

1. In the Object Explorer, in the YugoNationalBank node, click Database Diagrams
2. A dialog box will inform you that this database doesn't have a diagram:
    
   
   
   Read the message and click Yes
3. Right-click Database Diagrams and click New Database Diagram
4. In the Add Table dialog box, click Customers and click the Add > button
5. Double-click CheckCashing to add it
6. In the same way, add the AccountTypes , Deposits, Employees, Locations, and Withdrawals tables
7. On the Add Table dialog box, click Close.
   Notice that, based on how we created the database and its objects, the relationships have been created already:
    
   
8. To save the diagram, on the Standard toolbar, click Save
9. Set its name to dgmYugoNationalBank and click OK
10. Close the window

On a typical database, information comes and goes. For a bank, customers accounts are created and deleted on a regular basis. When an account is deleted, there is concern about the transactions related to that account. Referential integrity allows you to manage these aspects of a database. You need to make sure that when data is deleted from a parent table, the child tables are notified and their related records are deleted also. When information is changed on a parent table, the related information is changed in the child tables.

To manage referential integrity, you use the Foreign Key Relationships dialog box. You can access it from the design view of a table or from the diagram window.

1. In the Object Explorer, expand the Tables node under YugoNationalBank.
   Right-click dbo.Customers and click Design
2. Right-click in the table and click Relationships
3. Under Selected Relationships, click FK_CustomerAccount. In the right section, expand INSERT And UPDATE Specification
4. Click Delete Rule. In its combo box, select Cascade
5. Click Update Rule. In its combo box, select Cascade:
    
   
6. In the same way, specify the following
    

   Foreign Key	Delete Rule	Update Rule
   FK_ChargeReasons	Cascade	Cascade
   FK_Customers	Cascade	Cascade
   FK_Depositor	Cascade	Cascade
   FK_TypeOfAccount	Cascade	Cascade

7. Click Close
8. Save and close the table
9. In the same way, open the Deposits table in design view
10. Access its Relationships dialog box
11. Access the properties of its FK_Clerk field.
    Specify its Delete Rule and its Update Rule both to Cascade
12. Perform the same actions for the other relationships
13. Close the Relationships dialog box
14. Save and close the table