Quiz for Correlated Subquery

Q1: What makes a subquery correlated?

A. It runs independently B. It references columns from outer query C. It uses aggregation D. It uses JOIN

Q2: What distinguishes a correlated subquery from a regular subquery?

A. It returns multiple rows B. It depends on outer query values C. It can’t use WHERE clause D. It runs before the main query

Q3: Where is a correlated subquery commonly used?

A. In SELECT clause B. In FROM clause C. In WHERE clause D. In ORDER BY clause

Q4: What does a correlated subquery return?

A. Single value B. Multiple rows C. Depends on outer query D. No rows

Q5: Which SQL clause most often includes correlated subqueries?

A. GROUP BY B. WHERE C. ORDER BY D. LIMIT

Q6: Can correlated subqueries be used in UPDATE statements?

A. Yes B. No C. Only in MySQL D. Only in PostgreSQL

Q7: How many times does a correlated subquery execute?

A. Once for entire query B. Once per outer row C. Depends on data type D. Twice

Q8: Which keyword is necessary for writing a correlated subquery?

A. JOIN B. EXISTS C. GROUP BY D. None

Q9: Which is more efficient generally?

A. Correlated subquery B. Non-correlated subquery C. JOIN with subquery D. It depends on index

Q10: What happens if a correlated subquery does not find any match?

A. Returns NULL B. Skips the row C. Throws error D. Returns all rows

Q11: Which clause cannot have a correlated subquery directly?

A. WHERE B. FROM C. SELECT D. HAVING

Q12: Which operator is commonly used with correlated subqueries?

A. = B. IN C. EXISTS D. BETWEEN

Q13: Which DBMS supports correlated subqueries?

A. Only Oracle B. Only MySQL C. Only PostgreSQL D. All major DBMS

Q14: What does the term "correlation" imply in subqueries?

A. Referencing outside variables B. Subquery joins itself C. Recursion is used D. All rows are returned

Q15: What is the result of a correlated subquery if no rows match?

A. NULL B. Default value C. No row in result D. 0

Q16: What is the impact of correlated subqueries on performance?

A. They are faster B. They can be slow due to repeated execution C. They are cached D. They are optimized away

Q17: Which example represents a correlated subquery?

A. SELECT * FROM emp WHERE sal > (SELECT AVG(sal) FROM emp); B. SELECT * FROM emp WHERE EXISTS (SELECT 1 FROM dept WHERE emp.deptno = dept.deptno); C. SELECT * FROM emp; D. SELECT MAX(sal) FROM emp;

Q18: Which clause can return different results for each row using correlated subqueries?

A. GROUP BY B. WHERE C. SELECT D. LIMIT

Q19: Which of these improves correlated subquery performance?

A. Using indexes B. Avoiding WHERE clause C. Using HAVING only D. Using random()

Q20: Are correlated subqueries allowed in DELETE statements?

A. Yes B. No C. Only in SELECT D. Only in INSERT

Q21: What is a common use case of correlated subqueries?

A. Sorting data B. Getting max value C. Filtering rows based on related data D. Summing values