Which relational algebra operator combines tuples from two relations based on a common attribute?

The correct answer is Option 3) Join.

Key Points

• Join is a relational algebra operator that combines tuples (rows) from two relations (tables) based on a common attribute.
• It merges rows from both relations that satisfy a specified condition — typically where values of the common attribute(s) are equal.
• There are several types of joins, such as:
  • Natural Join – joins on all common attributes automatically.
  • Theta Join – joins using a condition (like =, <, >, etc.).
  • Equi Join – a type of theta join using only the equality operator (=).
  • Outer Join – includes unmatched rows from one or both relations.
• Join operations are essential for reconstructing data that has been normalized into multiple tables.

Additional Information

• Option 1 – Union: Combines all tuples from two relations, eliminating duplicates. Both relations must be union-compatible (same number and type of attributes).
• Option 2 – Intersection: Returns only those tuples that are present in both relations. Also requires union-compatibility.
• Option 4 – Projection: Selects specific columns (attributes) from a relation, removing duplicates.
• Join is the only operator among the options that uses a common attribute to combine related rows from two relations.