Microsoft SQL Server - Lesson 20: Databases Constraints

	Databases Constraints

The Primary Key Constraint

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.

The transactions among the various objects of a database should make sure information of one object is accessible to another object. The objects that hold information, as we have mentioned already, are the tables.

To manage the flow of information from one table (A) to another table (B), the table that holds the information, A, must make it available to other tables, such as B. There are various issues that must be dealt with:

You must be able to uniquely identify each record from a table (A) without any confusion. For example, if you create a list of cars on a table, you should make sure that there is a unique (no duplicate) tag number for each car because each car should have one and must have one tag number. This ensures that there are no duplicate records on the table.
A table (A) that holds information should make that information available to other tables (such as B)
Two tables must not serve the same purpose. Once you have unique information on each table, one table can make its data available to other tables that need it so that the same information should not be entered in more than one table

These problems are solved by specifying a particular column as the "key" of the table. Such a column is referred to as the primary key.

In a relational database, which is the case for most of the databases you will be creating, each table should have at least one primary key. As an example, a primary key on an Account table of a bank database can be set on a Bank Account field because each customer should have a unique bank account number. A table can also use more than one column to represent the primary key if you judge it necessary.

Once you have decided that a table will have a primary key, you must decide what type of data that field will hold. If you are building a table that can use a known and obvious field as unique, an example would be the shelf number of a library, you can set its data type as char or varchar and make it a primary key. In many other cases, for example if you cannot decide on a particular field that would hold unique information, an example would be customers Contact Name, you should create your own unique field and make it the Primary Key. Such a field should have an int data type.

Practical Learning: Introducing Relationships

Start Microsoft SQL Server and connect to the server
Right-click the name of the server and click New Query

To start a new database, type the following code:

-- =============================================
-- 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

Visually Creating a Primary Key

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:

Creating a Primary Key With SQL

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 nvarchar(20),
    LastName nvarchar(20) NOT NULL
);

The Primary Key Constraint

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 nvarchar(20),
    LastName nvarchar(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 nvarchar(20),
    LastName nvarchar(20) NOT NULL,
    CONSTRAINT PK_Persons PRIMARY KEY(PersonID)
);
GO

The Foreign Key Constraint

Introduction

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.

Creating a Foreign Key in the Table Design View

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.

Creating a Foreign Key in the Relationships Dialog Box

To create a foreign key in a table:

From the Object Explorer, open the child table in Design View
Right-click anywhere in the table and click Relationships...
In the Foreign Key Relationships dialog box, click Add
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:
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

Creating a Foreign Key in SQL

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 nvarchar(20),
    LastName nvarchar(20) NOT NULL,
    GenderID int NULL FOREIGN KEY REFERENCES Genders(GenderID)
);

The Foreign Key Constraint

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 nvarchar(20),
    LastName nvarchar(20) NOT NULL,
    GenderID int NULL CONSTRAINT FKGenders
                       FOREIGN KEY REFERENCES Genders(GenderID)
);

Establishing a Relationship

Introduction

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.

Creating a Relationship

To create a relationship between two tables

Open the child table in the design view
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.
Under Selected Relationships, click the foreign key that will hold the relationship
In the right side, expand Tables And Columns Specification
Click its ellipsis button
In the Primary Key Table combo box, select the parent table that holds the primary data
Under the parent table, click and select its primary key column
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:
Click OK.
When a relationship has been created, it would show in the Tables And Column Specification section:
In the same way, you can create other relationships by clicking Add and configuring the link.
Once you have finished, click Close

Practical Learning: Creating Relationships

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(N'dbo.Locations', N'U') IS NOT NULL
  DROP TABLE dbo.Locations
GO

CREATE TABLE Locations
(
    LocationID int Identity(1,1) NOT NULL, 
    LocationCode nvarchar(10) NOT NULL,
    Address nvarchar(120),
    City nvarchar(50),
    State nvarchar(50),
    Notes text NULL, 
    CONSTRAINT PK_Locations PRIMARY KEY (LocationID)
);
GO
-- =========================================
-- Database: YugoNationalBank
-- Table:    AccountTypes
-- =========================================
USE YugoNationalBank
GO

IF OBJECT_ID(N'dbo.AccountTypes', N'U') IS NOT NULL
  DROP TABLE dbo.AccountTypes
GO

CREATE TABLE AccountTypes
(
    AccountTypeID int Identity(1,1) NOT NULL, 
    AccountType nvarchar(40) NOT NULL, 
    Notes text NULL, 
    CONSTRAINT PK_AccountTypes PRIMARY KEY (AccountTypeID)
);
GO

-- =========================================
-- Database: YugoNationalBank
-- Table:    Employees
-- =========================================
IF OBJECT_ID(N'dbo.Employees', N'U') IS NOT NULL
  DROP TABLE dbo.Employees
GO
CREATE TABLE Employees
(
    EmployeeID int identity(1,1) NOT NULL, 
    EmployeeNumber char(6),
    FirstName nvarchar(32),
    LastName nvarchar(32) NOT NULL,
    Title nvarchar(50),
    CanCreateNewAccount bit,
    HourlySalary smallmoney,
    EmailAddress nvarchar(100),
    Username nvarchar(20),
    Password nvarchar(20),
    Notes text,
    CONSTRAINT PK_Employees PRIMARY KEY (EmployeeID)
);
GO

-- =========================================
-- Database: YugoNationalBank
-- Table:    Customers
-- =========================================
IF OBJECT_ID(N'dbo.Customers', N'U') IS NOT NULL
  DROP TABLE dbo.Customers
GO
CREATE TABLE Customers
(
    CustomerID int Identity(1,1) NOT NULL,
    DateCreated date,
    AccountNumber nvarchar(20),
    AccountTypeID int Constraint FK_TypeOfAccount 
		References AccountTypes(AccountTypeID),
    CustomerName nvarchar(50) NOT NULL,
    DateUpdated date,
    Notes text, 
    CONSTRAINT PK_Customers PRIMARY KEY (CustomerID)
);
GO

-- =========================================
-- Database: YugoNationalBank
-- Table:    ChargeReasons
-- =========================================
IF OBJECT_ID(N'dbo.ChargeReasons', N'U') IS NOT NULL
  DROP TABLE dbo.ChargeReasons
GO
-- =========================================
-- Database: YugoNationalBank
-- Table:    Deposits
-- =========================================
IF OBJECT_ID(N'dbo.Deposits', N'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 date NOT NULL,
    DepositAmount smallmoney NOT NULL,
    Notes text, 
    CONSTRAINT PK_Deposits PRIMARY KEY (DepositID)
);
GO

-- =========================================
-- Database: YugoNationalBank
-- Table:    Withdrawals
-- =========================================
IF OBJECT_ID(N'dbo.Withdrawals', N'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 date 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(N'dbo.CheckCashing', N'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 date NOT NULL,
    CheckCashingAmount smallmoney NOT NULL,
    CheckCashingSuccessful bit NOT NULL,
    Notes text, 
    CONSTRAINT PK_CheckCashing PRIMARY KEY(CheckCashingID)
);
GO

Press F5 to execute the statement

Diagrams

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

In the Object Explorer, in the database node, you can click Database Diagrams
A dialog box will inform you that this database doesn't have a diagram. Read the message and click Yes
Right-click Database Diagrams and click New Database Diagram
In the Add Table dialog box, click each table and click the Add.
Alternatively, you can double-click a table to add it
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:
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:
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
Under the parent table, select its primary column
Under Foreign Table, select the foreign key column. Here is an example:
Once you are ready, click OK. A link would be created between the tables
In the same way, you can create other relationships.
When you have finished, you can save and close the database

Practical Learning: Creating a Diagram

In the Object Explorer, in the YugoNationalBank node, click Database Diagrams
A dialog box will inform you that this database doesn't have a diagram:

Read the message and click Yes
Right-click Database Diagrams and click New Database Diagram
In the Add Table dialog box, click Customers and click the Add > button
Double-click CheckCashing to add it
In the same way, add the AccountTypes , Deposits, Employees, Locations, and Withdrawals tables
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:
To save the diagram, on the Standard toolbar, click Save
Set its name to dgmYugoNationalBank and click OK
Close the window

Referential Integrity

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.

Practical Learning: Managing Referential Integrity

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

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

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

Check Constraints

Introduction

When performing data entry, in some columns, even after indicating the types of values you expect the user to provide for a certain column, you may want to restrict a range of values that are allowed. In the same way, you can create a rule that must be respected on a combination of columns before the record can be created. For example, you can ask the database engine to check that at least one of two columns received a value. For example, on a table that holds information about customers, you can ask the database engine to check that, for each record, either the phone number or the email address of the customer is entered.

The ability to verify that one or more rules are respected on a table is called a check constraint. A check constraint is a Boolean operation performed by the SQL interpreter. The interpreter examines a value that has just been provided for a column. If the value is appropriate:

The constraint produces TRUE
The value gets accepted
The value is assigned to the column

If the value is not appropriate:

The constraint produces FALSE
The value gets rejected
The value is not assigned to the column

You create a check constraint at the time you are creating a table.

Visually Creating a Check Constraint

To create a check constraint, when creating a table, right-click anywhere in (even outside) the table and click Check Constraints...

Check Constraints

This would open the Check Constraints dialog box. From that window, you can click Add. Because a constraint is an object, you must provide a name for it. The most important piece of information that a check constraint should hold is the mechanism it would use to check its values. This is provided as an expression. Therefore, to create a constraint, you can click Expression and click its ellipsis button. This would open the Check Constraint Expression dialog box.

To create the expression, first type the name of the column on which the constraint will apply, followed by parentheses. In the parentheses, use the arithmetic and/or SQL operators we studied already. Here is an example that will check that a new value specified for the Student Number is greater than 1000:

Check Constraint Expression

After creating the expression, you can click OK. If the expression is invalid, you would receive an error and given the opportunity to correct it.

You can create as many check constraints as you judge necessary for your table:

Check Constraints

After creating the check constraints, you can click OK.

Programmatically Creating a Check Constraint

To create a check constraint in SQL, first create the column on which the constraint will apply. Before the closing parenthesis of the table definition, use the following formula:

CONSTRAINT name CHECK (expression)

The CONSTRAINT and the CHECK keywords are required. As an object, make sure you provide a name for it. Inside the parentheses that follow the CHECK operator, enter the expression that will be applied. Here is an example that will make sure that the hourly salary specified for an employee is greater than 12.50:

CREATE TABLE Employees
(
	[Employee Number] nchar(7),
	[Full Name] varchar(80),
	[Hourly Salary] smallmoney,
	CONSTRAINT CK_HourlySalary CHECK ([Hourly Salary] > 12.50)
);

It is important to understand that a check constraint it neither an expression nor a function. A check constraint contains an expression and may contain a function as part of its definition.

After creating the constraint(s) for a table, in the Object Explorer of Microsoft SQL Server Management Studio, inside the table's node, there is a node named Constraints and, if you expand it, you would see the name of the constraint.

With the constraint(s) in place, during data entry, if the user (or your code) provides an invalid value, an error would display. Here is an example:

Instead of an expression that uses only the regular operators, you can use a function to assist in the checking process. You can create and use your own function or you can use one of the built-in Transact-SQL functions.

Practical Learning: Creating a Check Constraint

In the Object Explorer, right-click the name of the server and click Start PowerShell
In the PowerShell window, type SQLCMD and press Enter

To create a database and a new table that has a check mechanism, type the following:

CREATE DATABASE GreensvilleElectricCompany;
GO

CREATE TABLE Customers
(
    AccountNumber nchar(14) primary key not null,
    CustomerName nvarchar(50) NOT NULL,
    PhoneNumber nchar(16),
    EmailAddress nvarchar(50),
    CONSTRAINT CK_CustomerContact
	CHECK ((PhoneNumber IS NOT NULL) OR (EmailAddress IS NOT NULL))
);
GO

To add records to the new table, type the following:

INSERT INTO Customers
VALUES(N'188395805',N'Ann Zeke',N'301-128-3506',N'azeke@yahoo.jp'),
      (N'806200422',N'Joan Simms',N'410-114-6820',N'jsimmson@emailct.com'),
      (N'402628475',N'Peter Dorka',N'(202) 050-1629',N'pdorka@hotmail.com'),
      (N'666204275',N'James Bengbiss',N'443-158-1833',N'jbango@emailct.com');
GO

To try adding a new record to the table, type the following:

1> INSERT INTO Customers
2> VALUES(N'402628475',N'James Darfield',N'202-188-8742',N'');
3> GO

Notice that you receive an error:

Msg 2627, Level 14, State 1, Server CENTRAL, Line 1
Violation of PRIMARY KEY constraint 'PK__Customer__BE2ACD6E51BA1E3A'. Cannot ins
ert duplicate key in object 'dbo.Customers'.
The statement has been terminated.
1>

To delete the database, type:

DROP DATABASE GreensvilleElectricCompany;
GO

Type exit and press Enter
To close the PowerShell window, type exit and press Enter


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