We claim that delete anomality is due to table not being in 3NF, but...

Uh, I've been away for the weekend but some new questions about normalization non the less popped up ( it seems every time I give a thought about the subject, a new question arises ). I hope you don't mind if I ask some more, cause there's no way I can figure out this stuff on my own

1. Third Normal Form prohibits transitive dependencies ( non_key attribute CLASSROOM is dependent on other non key attribute TEACHER_NAME). One of the arguments why we should put a table in 3NF form is because of delete anomalies, which for SUBJECT_TEACHER table means, that if we remove certain subject from the table, then all of the data about certain teacher could also be lost ( in cases where teacher teaches just one subject ).

SUBJECT_TEACHER( SUBJECT_ID, SUB_NAME, TEACHER_NAME, CLASSROOM ) But same argument could be used for the following table:

SUBJECT_TEACHER( SUBJECT_ID, SUB_NAME, TEACHER_NAME ) But this time we don’t have any non-key attribute in a table dependent on some other non-key attribute, and as such SUBJECT_TEACHER is in 3NF. But despite table being in 3NF, all of data ( name of the teacher ) about certain teacher could still be lost.

So how can we claim that delete anomality is due to table not being in 3NF, when it is obvious this situation ( in our case data about certain teacher being lost ) can arise even when table complies to third form?

2) Say we have a relation with attributes A, B and C, where A is also a primary key. I know that transitive dependency means that a non-key attribute ( say C ) is dependent on other non key attribute ( B ) .

• I also assume that with transitive dependency C is also ALWAYS dependent on A?!
• But is C always dependent on A simply because B is dependent on A or… I mean, B will be dependent on A regardless of whether we have C attribute, but I assume that is not the case with C?

thank you Received on Sun Oct 26 2008 - 21:51:08 CET