Consider the following SQL commands and their corresponding relational algebra expressions. Identify which of the given options is correct.
1. SQL Command:
SELECT *
FROM Students S
JOIN Enrollments E ON S.StudentID = E.StudentID;
Relational Algebra:

2. SQL Command:
SELECT S.StudentName, E.CourseID
FROM Students S
JOIN Enrollments E ON S.StudentID = E.StudentID
WHERE E.CourseID = 'CS101';
Relational Algebra:

3. SQL Command:
SELECT S.StudentName
FROM Students S
LEFT JOIN Enrollments E ON S.StudentID = E.StudentID;
Relational Algebra:
Which of the following options correctly matches the SQL commands with their relational algebra expressions?

Correct option is C

Let’s solve it. Given,
SQL Command 1:
· The SQL command performs an inner join between Students and Enrollments based on StudentID.
· Given Relational Algebra:

· Correct Relational Algebra:

· Correct as the selection condition after the Cartesian product effectively represents an inner join.
SQL Command 2:
· The SQL command selects StudentName and CourseID from joined Students and Enrollments, filtered where CourseID is 'CS101'.
· Given Relational Algebra:

· Correct Relational Algebra:

· Correct as it accurately translates the SQL join and selection operations.
SQL Command 3:
· The SQL command performs a left join between Students and Enrollments based on StudentID.
· Given Relational Algebra:

· Correct Relational Algebra:

· Incorrect as the given algebra does not properly represent a left outer join.
Key Points:
1. Relational Algebra Joins:
· Inner Join: ⋈
· Left Outer Join: (⟕)
· Cartesian Product: ×
2. SQL Commands and Relational Algebra:
· Correctly map SQL join operations to their equivalent relational algebra expressions.
· Ensure the use of appropriate symbols and operations.
3. Projection and Selection:
· Projection: π
· Selection: σ
Knowledge Booster:
1. Relational Algebra:
· A procedural query language that works with the relational model of data.
· It's the theoretical foundation for SQL and other relational databases.
2. SQL Joins:
· Joins are used to combine rows from two or more tables based on a related column between them.
· Understanding different types of joins (inner, outer, left, right) is crucial for effective database querying.