BPSC (Ministry of Home Affairs) Senior Computer Operator Exam: 13.09.2022 (ICT)প্রশ্ন ও সমাধান

Answer:

• Candidate key: any minimal set of attributes that can uniquely identify every tuple in a relation. A table may have several candidate keys (e.g., both StudentID and Email might each uniquely identify a student).
• Primary key: the one candidate key chosen by the designer as the table's main identifier. It cannot be NULL and must stay unique for every row.
• Foreign key: an attribute (or set of attributes) in one table that references the primary key of another table, enforcing a link between the two tables (referential integrity).

erDiagram
    DEPARTMENT ||--o{ EMPLOYEE : employs
    DEPARTMENT {
        int DeptID PK
        string DeptName
    }
    EMPLOYEE {
        int EmpID PK
        string Name
        int DeptID FK
    }

Key	Uniquely identifies rows in	Can be NULL?	Can repeat?	Example
Candidate key	Its own table	No	No	StudentID, Email (either could become PK)
Primary key	Its own table (the chosen one)	No	No	StudentID
Foreign key	Refers to another table's PK	Yes, if the relationship is optional	Yes, many rows can reference the same parent	DeptID in Employee, referencing Department.DeptID

Exam takeaway: every primary key is a candidate key, but not every candidate key is chosen as primary; a foreign key doesn't identify rows in its own table — it links to a primary key in another table to maintain referential integrity.