SQL Subquery vs JOIN

Choose a subquery or JOIN from the output grain and columns the question needs.

What & Why

A correlated EXISTS subquery answers an eligibility question without multiplying the outer rows, while a JOIN returns the matching child columns and therefore may produce several rows per parent.

The Queryflo examples intentionally have different grains: one row per eligible feature versus one row per positive feedback record. They demonstrate when each shape is appropriate, not interchangeable ways to return the same result.

See How It Works

BUSINESS QUESTION

Compare feature eligibility with the detailed positive-feedback rows behind it.

id	name	description	team	released_at	deprecated_at
1	Activation Checklist	Guides new users through setup	growth	2024-01-20	NULL
2	CSV Export	Exports report data	platform	2024-02-14	NULL
3	Invite Nudges	Prompts workspace collaboration	growth	2024-03-05	NULL
4	Legacy Dashboard	Original reporting surface	analytics	2023-08-10	2024-06-01

id	user_id	feature_id	rating	nps_score	comment	created_at
1	101	1	5	9	Clear and useful	2024-01-25 12:15:00+00
2	102	2	4	NULL	Export worked well	2024-02-20 15:40:00+00
3	103	1	4	7	Helpful setup flow	2024-03-18 09:05:00+00
4	104	3	3	NULL	Needs clearer timing	2024-04-08 17:30:00+00

Choose the SQL shape that matches the required table grainStep 1 of 2

WHERE EXISTS (positive product.feedback row for feature)

feature_id	feature	positive feedback exists
1	Activation Checklist	yes
2	CSV Export	yes
3	Invite Nudges	no
4	Legacy Dashboard	no

EXISTS returns one row per eligible feature and does not expose child detail.

EXAMPLE QUERY

-- One row per eligible feature
SELECT
  f.id AS feature_id,
  f.name
FROM product.features f
WHERE EXISTS (
  SELECT 1
  FROM product.feedback fb
  WHERE fb.feature_id = f.id
    AND fb.rating >= 4
)
ORDER BY f.id;

-- One row per positive feedback record
SELECT
  f.id AS feature_id,
  f.name,
  fb.rating,
  fb.created_at
FROM product.features f
JOIN product.feedback fb ON fb.feature_id = f.id
WHERE fb.rating >= 4
ORDER BY f.name, fb.created_at, fb.id;

This lesson's practice is part of Pro.

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Advanced business practice