[Solved] Why does a correlated subquery execute slower than a non-correlated subquery? — Ans: Because it runs once per outer query row, increasing execution time | SQL

SQL - Subqueries

Why does a correlated subquery execute slower than a non-correlated subquery?

ABecause it uses more memory than non-correlated subqueries

BBecause it cannot use indexes on the outer query

CBecause it runs once per outer query row, increasing execution time

DBecause it returns multiple rows instead of one

Step-by-Step Solution

Solution:

Step 1: Understand execution frequency
Correlated subqueries execute once for each row of the outer query, causing repeated work.
Step 2: Compare with non-correlated subqueries
Non-correlated subqueries run once, so they are generally faster.
Final Answer:
Because it runs once per outer query row, increasing execution time -> Option C
Quick Check:
Repeated execution per row slows correlated subqueries [OK]

Quick Trick: Repeated execution per row causes slower correlated subqueries [OK]

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Why does a correlated subquery execute slower than a non-correlated subquery?