Question

a student select course table.

 studentnumber   name     age     selected course   credit

    1             tom      23            math           5

    2              jim      20          computer         4

……..

if the table doesn’t fit for the 2NF, why and how to correct it. thank you.

Answer 1

It is not 2NF. This is:

Students:

studentnumber   name     age    

1               tom      23            
2               jim      20     

Courses:

coursenumber   name      credit

1              math      5
2              computer  4

Student_Courses:

studentnumber  coursenumber

1              1
2              2

Because you want to break out attributes which are not wholly dependent on the originally candidate key (e.g. course and studentnumber). Said attribues were only partially dependent on the the orignal candidate key (e.g. credit was dependent on course but not student. Age and name were dependent on student but not course.). For each partial dependency, you create a new table so that the attributes ARE wholly dependent on the new candidate/primary key.