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2NF is based on the concept of full functional dependency and the table must be in 1NF. A relation schema R is in 2NF if every nonprime attribute A in R is fully functional dependent on the primary key of R.
- Anomalies can occur when attributes are dependent on only part of a multi-attribute (composite) key.
- A relation is in second normal form when all non-key attributes are dependent on the whole key. That is, no attribute is dependent on only a part of the key.
- Any relation having a key with a single attribute is in second normal form.
Take the following table structure as an example
Here we should realize that there are two functional dependencies, to make this table 2NF these attribute must be removed and placed somewhere else.
- Course is functionally dependent on Student_ID and Sub_Code,
- Student_Name and Student_Add are functionally dependent on Student_ID