Page 21 - Modular_V2.0_SQL_Flipbook

CourseID INTEGER PRIMARY KEY, CourseName CHAR(50),
StudentID INTEGER, -- Foreign Key

FOREIGN KEY (StudentID) REFERENCES Students(StudentID)
);
In this example, the Students table has StudentID as its primary key which uniquely identifies each

student while the Courses table contains a StudentID column that acts as a foreign key as it is
referencing the StudentID in the Students table.
Composite Primary Key

When a table requires unique identification for each row but one column is unable to fulfill the
requirement of uniqueness, in that case two or more columns can be used for making a primary key.

CREATE TABLE Students
(
Student_name CHAR(30), Father_name CHAR(30), Marks INT, PRIMARY KEY( Student_
name, Father_name)
);

This creates a composite candidate key where name and father name together defines a unique
record in the table.
Unique Constraint

The UNIQUE constraint ensures that each value in a column is unique. The UNIQUE and PRIMARY
KEY constraints both ensure that a column or set of columns is unique. A UNIQUE constraint is
automatically added to a PRIMARY KEY constraint. A unique key however allows one NULL value in
the column, it is applied to.

For example, the following SQL command creates a UNIQUE constraint on the StudentID column
when the Students table is created:

CREATE TABLE Students
(
StudentID INTEGER, FirstName CHAR(30), LastName CHAR(30), Marks INT,
UNIQUE(StudentID)
);
Default Constraint

The DEFAULT constraint is used to specify a default value for a column. If no other value is specified,
the default value will be added to all new records.

For example, the following SQL command sets a DEFAULT value for the Marks column when the
Students table is created:
CREATE TABLE Students

(
StudentID INTEGER, FirstName CHAR(30), LastName CHAR(30), Marks INT DEFAULT
33
);

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Introduction to MySQL