What is a constraint?
In the previous chapter we have seen how to create a table using CREATE TABLE command.
Now we will understand how to define constraints. Constraints are used to implement standard
and business rules. Data integrity of the database must be maintained. In order to ensure
data has integrity we have to implement certain rules or constraints. As these constraints are
used to maintain integrity they are called as integrity constraints.
Standard rules:
Standard constraints are the rules related to primary key and foreign key. Every table must have a primary key. Primary key must be unique and not null. Foreign key must derive its values from corresponding parent key. These rules are universal and are called as standard rules.
Business rules :
These rules are related to a single application. For example, in a payroll application we may
have to implement a rule that prevents any row of an employee if salary of the employee is
less than 2000. Another example is current balance of a bank account
Must be greater than or equal to 500.
Once the constraints are created, Oracle server makes sure that the constraints are not
violated whenever a row is inserted, deleted or updated. If constraint is not satisfied then the
operation will fail.
Constraints are normally defined at the time of creating table. But it is also possible to add
constraints after the table is created using ALTER TABLE command. Constraints are stored in
the Data Dictionary (a set of tables which stores information regarding database).
Types of constraints :
Constraints can be given at two different levels. If the constraint is related to a single column
the constraint is given at the column level otherwise constraint is to be given at the table
level. Base on the where a constraint is given, constraint are of two types:
- Column Constraints
- Table Constraints
Column Constraint :
A constraint given at the column level is called as Column Constraint. It defines a rule for a
single column. It cannot refer to column other than the column at which it is defined. A typical
example is PRIMARY KEY constraint when a single column is the primary key of the table.
Table Constraint :
A constraint given at the table level is called as Table Constraint. It may refer to more than
one column of the table. A typical example is PRIMARY KEY constraint that is used to define
composite primary key. A column level constraint can be given even at the table level, but a
constraint that deals with more than one column must be given only at the table level.
The following is the syntax of CONSTRAINT clause used with CREATE TABLE and ALTER TABLE
commands.
[CONSTRAINT constraint]
{ [NOT] NULL
| {UNIQUE | PRIMARY KEY}
| REFERENCES [schema.] table [(column)]
[ON DELETE CASCADE]
| CHECK (condition) }
The following is the syntax of table constraint. :
[CONSTRAINT constraint]
{ {UNIQUE | PRIMARY KEY} (column [,column] ...)
| FOREIGN KEY (column [,column] ...)
{ {UNIQUE | PRIMARY KEY} (column [,column] ...)
| FOREIGN KEY (column [,column] ...)
REFERENCES [schema.] table [(column [,column] ...)]
[ON DELETE CASCADE]
| CHECK (condition) }
The main difference between column constraint and table constraint is that in table constraint
we have to specify the name of the column for which the constraint is defined whereas in
column constraint it is not required as constraint is given on immediately after the column.
Now let us understand sample table to be throughout this book. It is very important to
understand these tables to get the best out of this book. I have made these tables to be easy
to understand.
Sample tables:
The following are the sample tables used throughout the book. These tables store information
about course, batches and subject. There are six tables to store the required information by
typical training center.
Let us first understand the meaning of each table.
The following are the required tables of our application.
Table Name DescriptionCourses Contains the details of all the courses offered by the institute.
Faculty Contains the details of the faculty members of the institute.
Course_faculty This table contains information regarding which faculty can handle
which course. It also contains rating regarding how good a faculty
member is in handling a particular course. The rating is based on
previous experience of the faulty member with that course.
Batches Contains the information about all the batches. It contains information about all the batches that started and completed, on going and scheduled but not yet started.
Students Contains information about all the students. Each student is assigned a new roll number whenever he/she joins a new course.
Payments Information about all the payments made by students. A single student may pay course fee in multiple installments for a single course.
The following few tables will give the list of columns of each of the table given in table 1.
COURSES Table
Contains information related to each course. Each course is given a unique code called course code.
Column Name Data Type Description CCODE VARCHAR2(5) Course Code. This is the primary key of the table.
NAME VARCHAR(30) Name of the course.
DURATION NUMBER(3) Duration of the course in no. of working days.
FEE NUMBER(5) Course fee of the course.
PREREQUISITE VARCHAR2(100) Prerequisite knowledge to do the course.
The following are the required constraints of COURSES table.
- CCODE is primary key.
- FEE must be greater than or equal to 0.
- DURATION must be greater than or equal to 0.
FACULTY Table
Contains information about all the faculty members. Each faculty member is given a code called as FACCODE.
Column Name Data Type Description
FACCODE VARCHAR2(5) Faculty code. This is the primary key of the table.
NAME VARCHAR2(30) Name of the faculty.
QUAL VARCHAR2(30) Qualification of the faculty member.
EXP VARCHAR2(100) Experience of the faculty member.
The following are the constraints of FACULTY table.
- FACCODE is primary key.
COURSE_FACULTY table :
Contains information regarding which faculty member can take which course. A single faculty member may be capable of handling multiple courses. However, each member is given a grade depending on his expertise in handling the subject. The grade will be wither A, B or C.
Column Name Data Type Description
FACCODE VARCHAR2(5) Faculty code.
CCODE VARCHAR2(5) Course the faculty can handle.
GRADE CHAR(1) Rating of faculty’s ability to handle this particular code. A – Very good, B- Good,
C- Average.
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