[Solved] Why might a nested subquery inside a WHERE clause cause performance issues in large databases? — Ans: Because the subquery runs once for each row in the outer query | SQL

SQL - Subqueries

Why might a nested subquery inside a WHERE clause cause performance issues in large databases?

ABecause nested subqueries are not supported in SQL

BBecause subqueries always create temporary tables

CBecause WHERE clauses cannot use indexes

DBecause the subquery runs once for each row in the outer query

Step-by-Step Solution

Solution:

Step 1: Understand execution of nested subqueries
Correlated subqueries inside WHERE run the inner query for each row of the outer query.
Step 2: Identify performance impact
This repeated execution can slow down queries on large datasets.
Final Answer:
Because the subquery runs once for each row in the outer query -> Option D
Quick Check:
Repeated subquery execution causes slow performance [OK]

Quick Trick: Correlated subqueries run repeatedly, slowing queries [OK]

Common Mistakes:

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Why might a nested subquery inside a WHERE clause cause performance issues in large databases?