[Solved] Why might a subquery in the WHERE clause cause performance issues in SQL? — Ans: Because it may execute the subquery for each row in the outer query | SQL

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:

Step 1: Understand subquery execution
Some subqueries in WHERE are executed once, but correlated subqueries run for each outer row.
Step 2: Identify performance impact
Executing subquery repeatedly can slow down query performance significantly.
Step 3: Evaluate other options
Subqueries do not always create temporary tables, can use indexes, and do not require JOINs.
Final Answer:
Because it may execute the subquery for each row in the outer query -> Option C
Quick Check:
Correlated subqueries can cause repeated execution slowing queries [OK]

Quick Trick: Correlated subqueries run per row, causing slowdowns [OK]

Common Mistakes:

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