Correlated queries question

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Write a query that returns student_id and a correlated subquery that returns the matching average group-wise writing_scorefor each parental_education category. Alias the result of the correlated subquery as parental_education_avg_writing.

My Code:

SELECT student_id, 
(SELECT AVG(writing_score) 
 from performance p1 where p1.parental_education = p2.parental_education
 group by parental_education) as parental_education_avg_writing
 from performance p2;

My Quesiton:

Why do we need to equate p1.parental_education = p2.parental_education ?
Trying to understand the logic.

Thank you!

Try read this article it’d break it down further for you and if you still have doubts let me know