What is a correlated subquery and why can it be slow?
PICTURE THIS: GIT FLOW
Simple meaning
It is a subquery that references columns from the outer query, so it may run once per outer row.
WHY — SQL instead of guessing?
Why interviewers care about SQL:
who only read docs from people who shipped.
and tied to Backend work.
Name the idea, why it exists, then one short example.
End with when you use it and one common pitfall.
STEPS — What happens step by step?
Before you speak the answer, walk the interviewer through these steps:
- 1It is a subquery
that references columns from the outer query, so it may run once per outer row.
- 2That is an N+1
pattern in SQL.
- 3You often rewrite it
as a JOIN or a grouped derived table so the planner can hash or merge once.
- 4Give an example
One tiny concrete case you can say aloud.
- 5Common mistake
What juniors usually get wrong.
- 6Close
When you pick this over the alternative.
EXAMPLE — See it in action
Here's a short line you can speak, broken into clear beats:
Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.
Key takeaway
It is a subquery that references columns from the outer query, so it may run once per outer row. That is an N+1 pattern in SQL.