4.4. 3rd Normal Form

• The Primary Key must fully define all Non-Key columns

• Non-Key columns must not depend on any other Key

• All fields (columns) can be determined only by the key in the table and no other column

4.4.1. Problem

Table 4.15. assignments

id (PK)	firstname	lastname	year	mission
1	Melissa	Lewis	2035	Ares3
2	Mark	Watney	2035	Ares3
3	Rick	Martinez	2035	Ares3

Note that pair (2035, Ares3) repeats several times. Knowing the mission name we can determine the year. This should be replaced with a relation (FK) to the database which would store this pair at given primary key. The important fact here is that, if we want to change year of a Ares3 mission, let say it was delayed due to the budget constraints, we have to change it in several places. Leaving one record unmodified will lead to data inconsistency.

4.4.2. Solution

Table 4.16. assignments

id (PK)	firstname	lastname	mission_id (FK)
1	Melissa	Lewis	3
2	Mark	Watney	3
3	Rick	Martinez	3

Table 4.17. missions

id (PK)	year	mission
1	2031	Ares1
2	2032	Ares2
3	2035	Ares3

Now changing a year for a mission would result in changing it for all astronauts.

Figure 4.11. The Primary Key must fully define all Non-Key columns Image credit: [1]

4.4.3. Recap

Figure 4.12. If Primary Key fully defines all Non-Key columns and Non-Key columns does not depend on any other Key, the database achieved 3rd Normal Form. Image credit: [1]

4.4.4. References

[1] (1,2)

Lowgren, Jesper. Database Normalization 1NF 2NF 3NF. Year: 2021. Retrieved: 2022-02-05. URL: https://www.youtube.com/watch?v=SK4H5tTT6-M