Which relational algebra operation is used to retrieve rows that satisfy a given predicate?

• A. Selection
• B. Projection
• C. Join
• D. Difference

Answer: (A)

Filtering rows by a condition is what the selection operation does in relational algebra. It takes a relation and a predicate (a boolean expression over the attributes) and returns the subset of tuples that satisfy the predicate. For example, if you have a relation Employees with attributes such as Age, Dept, and Salary, applying a condition like Salary > 50000 yields σ_{Salary > 50000}(Employees). The result preserves all the columns of the original relation; you’d use projection afterwards if you want to keep only certain columns. This is different from projection, which reduces columns, and from join, which combines tuples from two relations based on a condition or common attributes, and from difference, which removes tuples present in another relation. So, the operation that retrieves rows meeting a given predicate is the selection.