what is ERD with components and example

    

What is ERD:

ERD   is   abbreviation   of    Entity  Relationship    Diagram.
An   entity    relationship      diagram  is  a     graphical  representation    of   an information system    that   shows  the   relationship    between  people   , objects ,places   ,   concepts   or   events    within   that   system.   An   ERD   is  a  data   modeling   technique   that   can  help   define    business   processes   and   can     be   used    as   the    foundation   for  a   relational  data base.

The Key Elements of an ERD

An ERD has three  Basic components:

Entity : 

An entity is a person,object,place,concept or anything within sytem. In  business domain terms, it’s  a  concept ,  or    glossary   level   term.  

In  relational database terms, it’s the table.

Attribute:

Each entity has attributes. Within   each  entity,  there  can  be  more  than  one  attribute .  Attributes  provide   detailed   information   about   the  concept.  In  a relational   database  ,   attributes    are   represented   by   the   fields  where  the   information   inside   a   record  is   held.

Relationships:

The   real  insight   from  this  type  of  diagram  comes  when   we  see  how  entities relate  to   one   another,  or   relationships .  Relationships  can  be  thought  of   as  verbs  that   link   two   or   more  nouns. Relationships   can   be   modeled numerically , using   the   multiplicity   syntax  from  a   class   diagram,  or  using  Crows  Foot   Notation.

There are three types of relationships:

• One-to-one:
•  Both    tables  can  have  only one  record on  either side  of  the  relationship. Each  primary  key  value  relates  to   only   one (or no) record in the related table.  
• For Example:They are like spouses—you may or may not be married,  but if you are, both  you   and  your  spouse   have  only  one  spouse.
•  Most one-to-one relationships are forced by business rules and don't flow naturally from the data. In the absence of such a rule, you can usually combine both tables into one table without breaking any normalization rule.

• One-to-many: 
• The primary key table contains only one record that relates to none, one, or many records in the related table.
• For Example: This relationship is similar to the one between you and a parent. You have only one mother, but your mother may have several children.

• Many-to-many:

• Each record in both tables can relate to any number of records (or no records) in the other table. For instance, if you have several siblings, so do your siblings (have many siblings). Many-to-many relationships require a third table, known as an associate or linking table, because relational systems can't directly accommodate the relationship.

              Example of ERD

                                        Inventory Management System