Jiban Bima Corporation — Assistant Programmer Written Test: 2018প্রশ্ন ও সমাধান

• Super Key: any set of one or more attributes that, taken together, can uniquely identify a row/tuple in a relation. A super key may contain extra, redundant attributes beyond what's strictly needed for uniqueness.
• Primary Key: a minimal super key — chosen by the database designer as the official identifier of a table. It must be unique, not-null, and irreducible (removing any attribute from it would break uniqueness).

Example — table Student(StudentID, Email, Name, DeptID):

Candidate super keys	Is it a Primary Key?
{StudentID}	Yes — minimal, unique, chosen as PK
{Email}	No (not chosen) — but qualifies as a candidate key
{StudentID, Name}	Super key, but NOT minimal (Name is redundant)
{StudentID, Email, Name, DeptID}	Super key (the whole row), but far from minimal

• Every Primary Key is a Super Key, but not every Super Key is a Primary Key.
• A table can have multiple candidate keys (minimal super keys); exactly one of them is chosen as the Primary Key, the rest remain candidate keys.

Exam takeaway: Super Key = any uniquely-identifying attribute set (can be redundant); Primary Key = the specific minimal super key chosen to identify rows in the table.