ENTITY RELATIONSHIP DIAGRAM

ENTITY RELATIONSHIP DIAGRAM IS A SPECIALIZED GRAPHIC THAT ILLUSTRATES

THE INTERRELATIONSHIPS BETWEEN ENTITIES IN A DATABASE.

SYMBOLS USED IN ENTITY RELATIONSHIP DIAGRAM

• DIAMOMD ARE NORMALLY USED TO REPRESENT RELATIONSHIPS .

• OVALS ARE USED TO REPRESENT ATTRIBUTES.• BOXES ARE COMMONLY USED TO REPRESENT

ENTITIES.

ONE-TO-ONE RELATIONSHIP

• ONE-TO-ONE RELATIONSHIPS OCCUR WHEN THERE IS EXACTLY ONE RECORD IN THE FIRST TABLE THAT CORRESPONDS TO EXACTLY ONE RECORD IN THE RELATED TABLE.

• EXAMPLE , This is a one-to-one relationship table design, a STOCK table contains exactly one record in STOCK_DETAIL table. Both tables have the same Stock Id as primary key. In STOCK_DETAIL table, Stock Id is the primary key and also a foreign key to STOCK table.

ONE-TO-MANY RELATIONSHIP

• ONE -TO-MANY RELATIONSHIPS OCCUR WHEN EACH RECORD IN TABLE A MAY HAVE MANY LINKED RECORDS IN TABLE B BUT EACH RECORD IN TABLE B MAY HAVE ONLY ONE CORRESPONDING RECORD IN TABLE A.

• EXAMPLE, STUDENT AND PHONE ENTITY. STUDENT CAN HAVE ANY NUMBER OF PHONE NUMBERS. TO CREATE THIS RELATIONSHIP YOU NEED TO HAVE A STUDENT, PHONE AND STUDENT_PHONE TABLE.

MANY-TO-MANY RELATIONSHIP

• MANY-TO-MANY RELATIONSHIP IS A RELATIONSHIP WHERE MULTIPLE ROWS FROM TABLE A CAN CORRESPOND TO MULTIPLE ROWS IN TABLE B.

• EXAMPLE, IS A SCHOOL WHERE TEACHERS TEACH STUDENTS. IN MOST SCHOOLS EACH TEACHER CAN TEACH MULTIPLE STUDENTS AND EACH STUDENT CAN BE TAUGHT BY MULTIPLE TEACHERS.