The SQL SELECT statement can perform which of the following relational operations on data in a table?

Introduction / Context:
The SQL SELECT statement is the workhorse of relational databases and corresponds closely to operations from relational algebra. Two of the most important operations are selection, which chooses specific rows, and projection, which chooses specific columns. This question tests your understanding of how SELECT relates to these concepts and what capabilities it provides in practical query writing.

Given Data / Assumptions:

• We are working with a single SQL SELECT statement on a table.
• Selection refers to choosing rows that satisfy a condition.
• Projection refers to choosing specific columns from the table.
• We must identify which operations SELECT can perform.

Concept / Approach:
In relational algebra, selection is often represented by a sigma operator and projection by a pi operator. In SQL, the SELECT clause and the WHERE clause together implement these operations. The list of columns after SELECT specifies projection, and the conditions in WHERE implement selection. Therefore a single SELECT statement can implement both operations at the same time, giving it strong expressive power for data retrieval.

Step-by-Step Solution:
Step 1: Consider a query like SELECT name, salary FROM employees WHERE department_id = 10. Step 2: The columns name and salary in the SELECT list represent projection, because they choose specific columns from the table. Step 3: The WHERE department_id = 10 clause represents selection, because it filters rows based on a condition. Step 4: Since a single SELECT statement can both project columns and select rows, it clearly performs both operations. Step 5: Protection and security are handled by user privileges and database administration, not by the SELECT statement itself.

Verification / Alternative check:
You can rewrite many relational algebra expressions that involve selection and projection as one SQL SELECT statement. For example, sigma condition (pi columns (Table)) is implemented by SELECT columns FROM Table WHERE condition. This mapping appears in textbooks and confirms that SELECT can perform both projection and selection.

Why Other Options Are Wrong:
Option A is incorrect because SELECT does more than projection; it also filters rows via WHERE, HAVING, and other clauses. Option B is incorrect because selection alone would mean filtering rows without choosing specific columns, which does not reflect full SELECT capabilities. Option C is incorrect because protection and security are not core functions of the SELECT statement; instead, they are managed by access control mechanisms and roles in the DBMS.

Common Pitfalls:
Some learners think of SELECT only as column selection because of the name, overlooking how WHERE and HAVING connect it to relational selection. Another pitfall is assuming that security is provided by the query itself, when in fact the database checks permissions before running the query. Recognizing that a single SELECT performs both projection and selection helps deepen your understanding of how SQL implements relational algebra.

Final Answer:
The SQL SELECT statement is capable of both projection and selection, as represented in option D.