Bird
0
0

Why might a subquery in the WHERE clause cause performance issues in SQL?

hard📝 Conceptual Q10 of 15
SQL - Subqueries
Why might a subquery in the WHERE clause cause performance issues in SQL?
ABecause subqueries cannot use indexes
BBecause subqueries always create temporary tables
CBecause it may execute the subquery for each row in the outer query
DBecause subqueries require explicit JOIN statements
Step-by-Step Solution
Solution:
  1. Step 1: Understand subquery execution

    Some subqueries in WHERE are executed once, but correlated subqueries run for each outer row.
  2. Step 2: Identify performance impact

    Executing subquery repeatedly can slow down query performance significantly.
  3. Step 3: Evaluate other options

    Subqueries do not always create temporary tables, can use indexes, and do not require JOINs.
  4. Final Answer:

    Because it may execute the subquery for each row in the outer query -> Option C
  5. Quick Check:

    Correlated subqueries can cause repeated execution slowing queries [OK]
Quick Trick: Correlated subqueries run per row, causing slowdowns [OK]
Common Mistakes:
MISTAKES
  • Assuming all subqueries create temp tables
  • Believing subqueries can't use indexes
  • Thinking JOINs are mandatory with subqueries

Want More Practice?

15+ quiz questions · All difficulty levels · Free

Free Signup - Practice All Questions
More SQL Quizzes