Composite Key chapter corrected

[YaroslavKazeev]

Why the composite key example violates 2NF
The composite key example in the chapter uses a table where the primary key is made up of two columns: student_number and school_id. In this structure, if there is a non-prime column (a column not part of the primary key), such as name, and it depends only on part of the composite key (for example, just student_number), this is called a partial dependency. 2NF requires that all non-prime columns depend on the whole primary key, not just a part of it.

In the example, if name depends only on student_number (and not on both student_number and school_id), then the table is not in 2NF. This is because knowing just the student_number gives you the name, regardless of the school_id. This violates the rule of 2NF, which says there should be no partial dependency of any column on a subset of a candidate key.