Unary Relational Operations in Relational Algebra
Unary operations are operations that are applied to a single relation (table). These are used to filter or manipulate data from one table without involving another relation.
1. SELECT (σ) — Selection
Definition:
- Retrieves rows (tuples) from a relation that satisfy a given condition.
- It is a horizontal subset of the relation.
Syntax:
σ<condition>(Relation)
Example:
σ age > 25 (Student)
Returns all students older than 25.
2. PROJECT (π) — Projection
Definition:
- Retrieves specific columns (attributes) from a relation.
- Removes duplicate rows automatically (since relations are sets).
Syntax:
π<attribute list>(Relation)
Example:
π name, age (Student)
Returns the name and age columns from the Student table, removing duplicates.
3. RENAME (ρ) — Renaming
Definition:
- Used to rename the relation or its attributes.
- Useful when dealing with self-joins or complex queries.
Syntax:
ρ NewName(Relation) OR ρ(NewName(attr1, attr2, ...))(Relation)
Example:
ρ S(Student)
Renames the Student relation to S.
ρ Temp(sid, sname)(Student)
Renames the relation as Temp and attributes as sid, sname.
Summary Table
| Operation | Symbol | Purpose | Example |
|---|---|---|---|
| SELECT | σ | Filters rows by condition | σ age > 25 (Student) |
| PROJECT | π | Chooses specific columns | π name, age (Student) |
| RENAME | ρ | Renames relation/attributes | ρ S(Student) |
Let me know if you want practice questions or visual examples for these operations.