Database Reference
In-Depth Information
not the primary key of the relation is known as transitively dependent. This is because the
field is functionally dependent on another field, whereas the other field is dependent on the
primary key.
3NF does the following:
1. The table must already be in the 2NF.
2. Eliminate transitive dependencies (that is, all the non-key attributes depend only
on the primary key).
3. Any kind derived data is allowed, such as total columns. Derived columns are
defined in terms of other columns, rather than in terms of specific attributes. 4. Form a new
table to contain any separated fields.
To transform a second normal form relation into third normal form, move
transitively dependent attributes to relations where they depend only on the primary key.
To move from second normal form to third normal form, inter-data dependencies have to
be removed. Every table is being examined. Is the value of data-item A dependent on the
value of data-item B, or vice versa? If so the relevant data-items are spitted off into a sep-
arate table. In previous example SubjectName is divided into Subject and TeachingStaff
tables. Here, TeacherID determines TeacherName. TeacherName is transitively dependent
on SubjectName. TeacherID is therefore asking to be a primary key. A separate table is
formed, called as TeachingStaff with TeacherID as the primary key. This is demonstrated
below:
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