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Intro to Constraints; NOT NULL and UNIQUE Constraints

Objectives

This lesson covers the following objectives:

  • Define the term “constraint” as it relates to data integrity
  • State when it is possible to define a constraint at the column level, and when it is possible at the table level
  • State why it is important to give meaningful names to constraints
  • State which data integrity rules are enforced by NOT NULL and UNIQUE constraints
  • Write a CREATE TABLE statement which includes NOT NULL and UNIQUE constraints at the table and column levels
  • Explain how constraints are created at the time of table creation

Purpose

  • If you think about it, how would society function without rules?
  • It is a rule to stop at a red traffic light?
  • Would it be safe to drive without this rule?
  • For databases, it is a rule that a foreign-key value cannot be entered without first entering a primary-key value.
  • What do you think would happen to a database if this rule wasn’t enforced?
  • A database is only as reliable as the data that is in it.
  • Constraints are used to prevent invalid data entry into tables.
  • Would it make sense to have negative salary values or six students with the same student ID or two tables that no longer reference each other?
  • Without rules, how could you trust the integrity of the database?
  • In the next three lessons, you will study how to create the constraints that enforce the “rules.”
  • You will also learn how to manage them and view constraint definitions in the data dictionary.

Constraints in General

  • So, what exactly is a constraint?
  • Think of constraints as database rules.
  • All constraint definitions are stored in the data dictionary.
  • Constraints prevent the deletion of a table if there are dependencies from other tables.
  • Constraints enforce rules on the data whenever a row is inserted, updated, or deleted from a table.
  • Constraints are important and naming them is also important.
  • Although you could name a constraint “constraint_1” or “constraint_2,” you’d soon find it difficult to distinguish one constraint from another and would end up redoing a lot of work.

Creating Constraints

  • Recall the SQL syntax for creating a table.
  • In the CREATE TABLE statement shown on the next slide, each column and its data type is defined.
  • You use the CREATE TABLE statement to establish constraints for each column in the table.
  • There are two different places in the CREATE TABLE statement that you can specify the constraint details:
    • At the column level next to the name and data type
    • At the table level after all the column names are listed
CREATE TABLE clients
(client_number NUMBER(4),
first_name VARCHAR2(14),
last_name VARCHAR2(13));
  • Note that the column-level simply refers to the area in the CREATE TABLE statement where the columns are defined.
  • The table level refers to the last line in the statement below the list of individual column names.
  • The five types of CONSTRAINT are:
    • NOT NULL constraints
    • UNIQUE constraints
    • PRIMARY KEY constraints
    • FOREIGN KEY constraints
    • CHECK constraints

Constraints at the Column Level

  • A column-level constraint references a single column.
  • To establish a column-level constraint, the constraint must be defined in the CREATE TABLE statement as part of the column definition.
  • Examine the following SQL statement that establishes a column-level constraint.
Constraints at the Column Level
  • CREATE TABLE clients
  • The name of the constraint is clients_client_num_pk.
  • It enforces the business rule that the client_number is the primary key of the clients table.

Creating Primary Key constraints will be covered in more detail in the next lesson.

Naming Constraints

  • Every constraint in the database has a name. When a constraint is created, it can be given a name, such as clients_client_num_pk, or given no name, in which case the system gives the constraint a name, such as SYS_C00585417.
  • A naming convention can be the combination of the tablename abbreviated and a column name abbreviated followed by the constraint abbreviation: table-name_column-name_constraint-type
  • If the reserved word CONSTRAINT is used in the CREATE TABLE definition, you must give the constraint a name. Constraint names are limited to 30 characters.

Naming Constraints at the Column Level

  • It is best to name constraints yourself because system generated names are not easy to recognize.
  • Look at this table definition:
CREATE TABLE clients
(client_number NUMBER(4),
last_name VARCHAR2(13),
email VARCHAR2(80));
  • According to our naming convention:
    • A primary key constraint on client_number would be named clients_client_number_pk
    • A not null constraint on last_name would be named clients_last_name_nn
    • A unique constraint on email address would be named clients_email_uk
Naming Constraints at the Column Level

Constraint Naming Example

  • This example shows both a user-named constraint and a system-named constraint:
Constraint Naming Example
  • Two constraints have been created:
    • A user-named constraint named clients_client_num_pk, to enforce the rule that client_number is the primary key
    • A system-named constraint named SYS_Cn (where n is a unique integer) to enforce the rule that last_names cannot be null.

Constraints at the Table Level

  • Table-level constraints are listed separately from the column definitions in the CREATE TABLE statement.
  • Table-level constraint definitions are listed after all the table columns have been defined.
  • In the example shown, the unique constraint is listed last in the CREATE TABLE statement.
Constraints at the Table Level

Basic Rules for Constraints

  • Constraints that refer to more than one column (a composite key) must be defined at the table level
  • The NOT NULL constraint can be specified only at the column level, not the table level
  • UNIQUE, PRIMARY KEY, FOREIGN KEY, and CHECK constraints can be defined at either the column or table level
  • If the word CONSTRAINT is used in a CREATE TABLE statement, you must give the constraint a name

A constraint which refers to two columns cannot be defined as part of a column definition. Which of the two column definitions would we put it alongside? It couldn’t be both!

A table level constraint must be user-named, because table level constraints must include the keyword CONSTRAINT.

NOT NULL constraints cannot be defined at the table level because the ANSI/ISO SQL standard forbids it

Examine the Violations

CREATE TABLE clients(
client_number NUMBER(6),
first_name VARCHAR2(20),
last_name VARCHAR2(20),
phone VARCHAR2(20) CONSTRAINT phone_email_uk
UNIQUE(email,phone),
email VARCHAR2(10) CONSTRAINT NOT NULL,
CONSTRAINT emailclients_email NOT NULL,
CONSTRAINT clients_client_num_pk PRIMARY KEY (client_number));
Basic Rules for Constraints

Five Types of Constraints

  • Five types of constraints exist within an Oracle database.
  • Each type enforces a different rule.
  • The types are:
    • NOT NULL constraints
    • UNIQUE constraints
    • PRIMARY KEY constraints
    • FOREIGN KEY constraints
    • CHECK constraints
  • In the remainder of this lesson, you will learn about NOT NULL and UNIQUE constraints.
  • The next lesson will teach you about the other three types of constraints.

NOT NULL Constraint

  • A column defined with a NOT NULL constraint requires that for every row entered into the table, a value must exist for that column.
  • For example, if the email column in an employees table was defined as NOT NULL, every employee entered into the table MUST have a value in the email column.
  • When defining NOT NULL columns, it is customary to use the suffix _nn in the constraint name.
  • For example, the constraint name for the NOT NULL email column in the employees table could be emp_email_nn.

UNIQUE Constraint

  • A UNIQUE constraint requires that every value in a column or set of columns (a composite key) be unique; that is, no two rows of a table can have duplicate values.
  • For example, it may be important for a business to ensure that no two people have the same email address.
  • The email column could be defined using a UNIQUE constraint.
  • The column or set of columns that is defined as UNIQUE is called a unique key.
  • If the combination of two or more columns must be unique for every entry, the constraint is said to be a composite unique key.
  • Stating that all combinations of email and last name must be UNIQUE is an example of a composite unique key.
  • The word “key” refers to the columns, not the constraint names.

Unique Constraint Example

  • If the email column in the table is defined with a UNIQUE constraint, no other client entry can have an identical email.
  • What if two clients live in the same household and share an email address?
Unique Constraint Example

The benefit of naming constraints using a convention is demonstrated here. The name of the constraint being violated is displayed as an error message, that tells the user what the problem is. If the constraint had not been named, the system generated name would be displayed, making it much more difficult for the user to resolve the issue.

Defining UNIQUE Constraints

  • When defining UNIQUE constraints, it is customary to use the suffix _uk in the constraint name.
  • For example, the constraint name for the UNIQUE email column in the employees table could be emp_email_uk.
  • To define a composite unique key, you must define the constraint at the table level rather than the column level.
  • An example of a composite unique-key constraint name is:
CONSTRAINT clients_phone_email_uk UNIQUE(email,phone)

Composite Unique Key

  • UNIQUE constraints allow the input of nulls unless the column also has a NOT NULL constraint defined.
  • A null in a column (or in all columns of a composite unique key) always satisfies a UNIQUE constraint because nulls are not considered equal to anything.
Composite Unique Key

Constraints Created at Table Creation

  • When you add a NOT NULL constraint as part of a table creation statement, the Oracle database will create a Check Constraint in the database to enforce a value in the NOT NULL column.
  • This constraint creation can be almost invisible to you when you create your table—Oracle just does it.
  • At the end of your table creation statement, the message “Table created” displays, but no details are provided about the number or types of constraints that were also created.

Terminology

Key terms used in this lesson included:

  • CHECK constraint
  • Constraint
  • Column level constraint
  • FOREIGN KEY
  • NOT NULL constraints
  • PRIMARY KEY
  • Table level constraint
  • UNIQUE constraints
  • UNIQUE KEY

Summary

In this lesson, you should have learned how to:

  • Define the term “constraint” as it relates to data integrity
  • State when it is possible to define a constraint at the column level, and when it is possible at the table level
  • State why it is important to give meaningful names to constraints
  • State which data integrity rules are enforced by NOT NULL and UNIQUE constraints
  • Write a CREATE TABLE statement which includes NOT NULL and UNIQUE constraints at the table and column levels
  • Explain how constraints are created at the time of table creation

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