Which operation is used to extract specified columns from a table?

Concept

Relational algebra uses operators to transform relations. Projection is a unary operator, written π, that returns selected attributes of a relation; under set semantics, duplicate result tuples are removed.

Application

The task is to retain only specified columns from one table. That is the role of projection, represented here by the value Project.

Contrast

• Join combines compatible tuples from two input relations according to a condition.

• Extract is an informal data-retrieval verb, not the conventional relational-algebra operator name.

• Substitute describes replacing a value or expression, not a standard relational-algebra operator for this task.

Cross-check

For example, applying πname, department(Employee) produces a relation containing only the name and department attributes. Thus the required operation is Project.