One-to many relationships in ER diagram

database relational-database entity-relationship
yrazlik picture yrazlik · Apr 9, 2013 · Viewed 84.3k times · Source

I am trying to show the following in the ER diagram: 

There are instructors and courses, a course is taught by only one instructor
whereas an instructor can give many courses.

enter image description here

enter image description here

My question is, is there any difference between two diagrams, in other words, does it matter which line we turn into an arrow, or what only matters is only the direction of the arrow?

Also, if we think about the mapping cardinalities; is it 1 to many or many to 1? If we think in terms of courses, then it is many to one but if we think in terms of instructors, then it is one to many. How do we decide this? Thank you.

Answer

Dulantha picture Dulantha · Apr 9, 2013

In ER diagrams when the relationship is denoted the arrows are not used. Some instructors use this arrow when they want to decide the cardinalities but that is just to get the cardinality (1:1, 1:M and N:M)

I have attached the ER diagram for this in Chen notation and also using Crow Notation you can use either of them.

ER Digram

Deciding the cardinality for a relationship is a practical scenario there is no hard and pass rule to obtain it. What you need to do is start from one side of the relationship and take one tuple (instance) and see how many tuples from the other entity participate for the relationship. Then do the vise versa. Then you know the participation number of tuples) from each entity to the relationship. Think about set theory and functions in mathematics when you decide the cardinality (ie Set of instructors, Set of Courses and set of Teaches relationship type) then this is so easy but if you are not from a mathematic background just think of practical scenario.

For Example

a) For 1 instructor he or she can teach Many (M) courses

b) For 1 Course there is only 1 instructor

so in instructor side there is always 1 in a) and b) but in Courses there is M and 1 in a) and b) there for Instructor:Course cardinality is 1:M

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