Home > Articles

This chapter is from the book

Integrity Constraints

Integrity constraints are used to ensure accuracy and consistency of data in a relational database. Data integrity is handled in a relational database through the concept of referential integrity. There are many types of integrity constraints that play a role in referential integrity (RI).

Primary Key Constraints

Primary key is the term used to identify one or more columns in a table that make a row of data unique. Although the primary key typically consists of one column in a table, more than one column can comprise the primary key. For example, either the employee's Social Security number or an assigned employee identification number is the logical primary key for an employee table. The objective is for every record to have a unique primary key or value for the employee's identification number. Because there is probably no need to have more than one record for each employee in an employee table, the employee identification number makes a logical primary key. The primary key is assigned at table creation.

The following example identifies the EMP_ID column as the PRIMARY KEY for the EMPLOYEES table:

CREATE TABLE EMPLOYEE_TBL
(EMP_ID    CHAR(9)    NOT NULL PRIMARY KEY,
EMP_NAME    VARCHAR (40)  NOT NULL,
EMP_ST_ADDR  VARCHAR (20)  NOT NULL,
EMP_CITY    VARCHAR (15)  NOT NULL,
EMP_ST     CHAR(2)    NOT NULL,
EMP_ZIP    INTEGER(5)   NOT NULL,
EMP_PHONE   INTEGER(10)   NULL,
EMP_PAGER   INTEGER(10)   NULL);

This method of defining a primary key is accomplished during table creation. The primary key in this case is an implied constraint. You can also specify a primary key explicitly as a constraint when setting up a table, as follows:

CREATE TABLE EMPLOYEE_TBL
(EMP_ID    CHAR(9)    NOT NULL,
EMP_NAME    VARCHAR (40)  NOT NULL,
EMP_ST_ADDR  VARCHAR (20)  NOT NULL,
EMP_CITY    VARCHAR (15)  NOT NULL,
EMP_ST     CHAR(2)    NOT NULL,
EMP_ZIP    INTEGER(5)   NOT NULL,
EMP_PHONE   INTEGER(10)   NULL,
EMP_PAGER   INTEGER(10)   NULL,
PRIMARY KEY (EMP_ID));

The primary key constraint in this example is defined after the column comma list in the CREATE TABLE statement.

A primary key that consists of more than one column can be defined by either of the following methods:

CREATE TABLE PRODUCTS
(PROD_ID    VARCHAR2(10)   NOT NULL,
 VEND_ID    VARCHAR2(10)   NOT NULL,
 PRODUCT    VARCHAR2(30)   NOT NULL,
 COST       NUMBER(8,2)	 NOT NULL,
PRIMARY KEY (PROD_ID, VEND_ID));
ALTER TABLE PRODUCTS
ADD CONSTRAINT PRODUCTS_PK PRIMARY KEY (PROD_ID, VEND_ID);

Unique Constraints

A unique column constraint in a table is similar to a primary key in that the value in that column for every row of data in the table must have a unique value. While a primary key constraint is placed on one column, you can place a unique constraint on another column even though it is not actually for use as the primary key.

Study the following example:

CREATE TABLE EMPLOYEE_TBL
(EMP_ID    CHAR(9)    NOT NULL   PRIMARY KEY,
EMP_NAME    VARCHAR (40)  NOT NULL,
EMP_ST_ADDR  VARCHAR (20)  NOT NULL,
EMP_CITY    VARCHAR (15)  NOT NULL,
EMP_ST     CHAR(2)    NOT NULL,
EMP_ZIP    INTEGER(5)   NOT NULL,
EMP_PHONE   INTEGER(10)   NULL     UNIQUE,
EMP_PAGER   INTEGER(10)   NULL);

The primary key in this example is EMP_ID, meaning that the employee identification number is the column that is used to ensure that every record in the table is unique. The primary key is a column that is normally referenced in queries, particularly to join tables. The column EMP_PHONE has been designated as a UNIQUE value, meaning that no two employees can have the same telephone number. There is not a lot of difference between the two, except that the primary key is used to provide an order to data in a table and, in the same respect, join related tables.

Foreign Key Constraints

A foreign key is a column in a child table that references a primary key in the parent table. A foreign key constraint is the main mechanism used to enforce referential integrity between tables in a relational database. A column defined as a foreign key is used to reference a column defined as a primary key in another table.

Study the creation of the foreign key in the following example:

CREATE TABLE EMPLOYEE_PAY_TBL
(EMP_ID      CHAR(9)    NOT NULL,
POSITION      VARCHAR2(15)  NOT NULL,
DATE_HIRE     DATE      NULL,
PAY_RATE      NUMBER(4,2)  NOT NULL,
DATE_LAST_RAISE  DATE      NULL,
CONSTRAINT EMP_ID_FK FOREIGN KEY (EMP_ID) REFERENCES EMPLOYEE_TBL (EMP_ID));

The EMP_ID column in this example has been designated as the foreign key for the EMPLOYEE_PAY_TBL table. This foreign key, as you can see, references the EMP_ID column in the EMPLOYEE_TBL table. This foreign key ensures that for every EMP_ID in the EMPLOYEE_PAY_TBL, there is a corresponding EMP_ID in the EMPLOYEE_TBL. This is called a parent/child relationship. The parent table is the EMPLOYEE_TBL table, and the child table is the EMPLOYEE_PAY_TBL table. Study Figure 3.2 for a better understanding of the parent table/child table relationship.

Figure 3.2 The parent/child table relationship.

In this figure, the EMP_ID column in the child table references the EMP_ID column in the parent table. In order for a value to be inserted for EMP_ID in the child table, there must first exist a value for EMP_ID in the parent table. Likewise, for a value to be removed for EMP_ID in the parent table, all corresponding values for EMP_ID must first be removed from the child table. This is how referential integrity works.

NOTE

The foreign key constraint is one area where MySQL does not comply with the ANSI standard. Foreign keys exist in MySQL, but are not used to enforce referential integrity as they are in most implementations such as Oracle.

A foreign key can be added to a table using the ALTER TABLE command, as shown in the following example:

alter table employee_pay_tbl
add constraint id_fk foreign key (emp_id)
references employee_tbl (emp_id);

NOTE

The options available with the ALTER TABLE command differ among different implementations of SQL, particularly when dealing with constraints. In addition, the actual use and definitions of constraints also vary, but the concept of referential integrity should be the same with all relational databases.

NOT NULL Constraints

Previous examples use the keywords NULL and NOT NULL listed on the same line as each column and after the data type. NOT NULL is a constraint that you can place on a table's column. This constraint disallows the entrance of NULL values into a column; in other words, data is required in a NOT NULL column for each row of data in the table. NULL is generally the default for a column if NOT NULL is not specified, allowing NULL values in a column.

Using Check Constraints

Check (CHK) constraints can be utilized to check the validity of data entered into particular table columns. Check constraints are used to provide back-end database edits, although edits are commonly found in the front-end application as well. General edits restrict values that can be entered into columns or objects, whether within the database itself or on a front-end application. The check constraint is a way of providing another protective layer for the data.

The following example illustrates the use of a check constraint:

CREATE TABLE EMPLOYEE_TBL
(EMP_ID    CHAR(9)    NOT NULL,
EMP_NAME    VARCHAR2(40)  NOT NULL,
EMP_ST_ADDR  VARCHAR2(20)  NOT NULL,
EMP_CITY    VARCHAR2(15)  NOT NULL,
EMP_ST     CHAR(2)    NOT NULL,
EMP_ZIP    NUMBER(5)   NOT NULL,
EMP_PHONE   NUMBER(10)   NULL,
EMP_PAGER   NUMBER(10)   NULL),
PRIMARY KEY (EMP_ID),
CONSTRAINT CHK_EMP_ZIP CHECK ( EMP_ZIP = '46234');

The check constraint in this table has been placed on the EMP_ZIP column, ensuring that all employees entered into this table have a ZIP code of '46234'. Perhaps that is a little restricting. Nevertheless, you can see how it works.

If you wanted to use a check constraint to verify that the ZIP code is within a list of values, your constraint definition could look like the following:

CONSTRAINT CHK_EMP_ZIP CHECK ( EMP_ZIP in ('46234','46227','46745') );

If there is a minimum pay rate that can be designated for an employee, you could have a constraint that looks like the following:

CREATE TABLE EMPLOYEE_PAY_TBL
(EMP_ID      CHAR(9)    NOT NULL,
POSITION      VARCHAR2(15)  NOT NULL,
DATE_HIRE     DATE      NULL,
PAY_RATE      NUMBER(4,2)  NOT NULL,
DATE_LAST_RAISE  DATE      NULL,
CONSTRAINT EMP_ID_FK FOREIGN KEY (EMP_ID) REFERENCES EMPLOYEE_TBL (EMP_ID),
CONSTRAINT CHK_PAY CHECK ( PAY_RATE > 12.50 ) );

In this example, any employee entered in this table must be paid more than $12.50 an hour. You can use just about any condition in a check constraint, as you can with a SQL query. You learn more about these conditions in later hours.

Dropping Constraints

Any constraint that you have defined can be dropped using the ALTER TABLE command with the DROP CONSTRAINT option. For example, to drop the primary key constraint in the EMPLOYEES table, you can use the following command:

ALTER TABLE EMPLOYEES DROP CONSTRAINT EMPLOYEES_PK;
Table altered.

Some implementations may provide shortcuts for dropping certain constraints. For example, to drop the primary key constraint for a table in Oracle, you can use the following command:

ALTER TABLE EMPLOYEES DROP PRIMARY KEY;
Table altered.

NOTE

Some implementations allow you to disable constraints. Instead of permanently dropping a constraint from the database, you may want to temporarily disable the constraint, and then enable it later.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020