Posted by: junkynfunky April 3, 2013
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Stupida is right
The general rule of thumb is nouns are entities and verbs/adverbs are attributes. There may be some exceptions to this rule according to your requirement. After you create boxes with entity as the title and their attributes( right below it), the relationship and cardinality should be pretty easy. Cardinality is the maximum and minimum constraint you put on the relationship. The main point is to consider the ramifications of those cardinalities.
Lets say you have a professor entity on left side and the students entity on the right side. So when you do cardinality, you place yourself on the professor box and look towards student, then ask how many students can a professor teach in a class? If the business rule says, there has to be minimum 10 students to offfer a class and max 32 students per class, the cardinality is (10,32).
Then you place yourself in the student box and look toward professor then ask how many professors teach a class of students. If your answer is 1, then the cardinality is 1. The ramification is when you offer a class, it must have a professor. DBMS does not let you leave the professor field null.
Lets say there is another entity called course. A student may be enrolled in one or more classes. Look at the verb may be. This means a student can take zero or more courses. It you put the (1,n) as the cardinality here, it means each student must have taken a class which creates some problem later. So the cardinality should be (0,n).
Hope this clears it for you.
The general rule of thumb is nouns are entities and verbs/adverbs are attributes. There may be some exceptions to this rule according to your requirement. After you create boxes with entity as the title and their attributes( right below it), the relationship and cardinality should be pretty easy. Cardinality is the maximum and minimum constraint you put on the relationship. The main point is to consider the ramifications of those cardinalities.
Lets say you have a professor entity on left side and the students entity on the right side. So when you do cardinality, you place yourself on the professor box and look towards student, then ask how many students can a professor teach in a class? If the business rule says, there has to be minimum 10 students to offfer a class and max 32 students per class, the cardinality is (10,32).
Then you place yourself in the student box and look toward professor then ask how many professors teach a class of students. If your answer is 1, then the cardinality is 1. The ramification is when you offer a class, it must have a professor. DBMS does not let you leave the professor field null.
Lets say there is another entity called course. A student may be enrolled in one or more classes. Look at the verb may be. This means a student can take zero or more courses. It you put the (1,n) as the cardinality here, it means each student must have taken a class which creates some problem later. So the cardinality should be (0,n).
Hope this clears it for you.