Key

• Super key: A set of one or more attributes that uniquely identify a row in a table. May contain extra (redundant) attributes.
• Candidate key: A minimal super key—no attribute can be removed without losing uniqueness. A table may have multiple candidate keys.
• Primary key: One chosen candidate key to uniquely identify rows in the table.
• Foreign key: An attribute (or set of attributes) in one table that refers to the primary key (or candidate key) of another table, creating a relationship between the two tables.

Example

Table: Student(ID, Name, Email, Phone)

• {ID}, {Email} → candidate keys (unique and minimal).
• {ID, Name} → super key (unique, but not minimal).
• {ID} → primary key (chosen candidate key).
• In another table Course(StudentID, CourseName), StudentID is a foreign key referencing Student(ID).

Relational Algebra

Select

• like select * from, where
• $\land\ (\text{and}),\ \lor\ (\text{or}),\ \lnot\ (\text{not})$
• The select predicate may include comparisons between two attributes

Project

• like select ID, name from