Independent practice · SQLite

Find conflicting refund keys

Practice dataset

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Your task

Return refund_id once for every key with conflicting order_id, amount, status or booked_at. Sort refund_id.
Exact copies alone are not conflicts. All source refund fields are non-NULL.

Pass every output check to complete this task. Suggested active time: 10 minutes. You can keep working after that.

Required output

Return exactly these columns in this order.

refund_idstring

Expected output: 1 rows. Keep the requested row order.

Values, data types and the number of copies of each row must match. A missing value is distinct from zero and from an empty string.

Preserve SQL NULL unless the task explicitly gives a replacement rule. Check JOINs for extra copies of rows.

Reconciliation checks

Every requested business key appears once.
Preserve SQL NULL and exact output column names.

Need a next step?

Hint 1: where to start

Group by refund_id.

Hint 2: build the method

Compare MIN and MAX for each non-key field.

Hint 3: check your work

Combine conflicting-field tests with OR in HAVING.

How your work is checked

Every output check must pass. The score shows how much matched and helps you find mistakes; completion comes from passing all checks. Active time is recorded separately.

Passing records a correct practice output on this device. It is not a professional certification.

Source materials

21 original source records.

Data dictionary

orders.order_id integer: Unique sale key; an unmatched refund must not create a sale.
orders.region text: Sales reporting region.
orders.status text: Only paid sales qualify.
orders.amount integer: Sale value in cents; includes zero.
orders.ordered_at text: ISO sale date; select the half-open reporting month.
refunds.refund_id text: Business key. Collapse exact repeats; exclude all conflicting variants.
refunds.order_id integer: May be orphaned or refer to a sale outside the cohort.
refunds.amount integer: Positive refunded cents; multiple distinct refunds may belong to one order.
refunds.status text: Only posted refunds reduce revenue.
refunds.booked_at text: ISO booking date; apply the same half-open reporting month.

View database setup and records

CREATE TABLE orders (order_id INTEGER, region TEXT, status TEXT, amount INTEGER, ordered_at TEXT);
INSERT INTO orders VALUES
(4000,'East','pending',1000,'2026-09-01'),
(4001,'West','paid',1125,'2026-10-01'),
(4002,'North','paid',1250,'2026-09-01'),
(4003,'East','paid',1375,'2026-09-04'),
(4004,'West','paid',0,'2026-09-05'),
(4005,'North','paid',1625,'2026-09-06'),
(4006,'East','paid',1750,'2026-09-07'),
(4007,'West','pending',1875,'2026-09-08');
CREATE TABLE refunds (refund_id TEXT, order_id INTEGER, amount INTEGER, status TEXT, booked_at TEXT);
INSERT INTO refunds VALUES
('R4000',4002,100,'requested','2026-09-01'),
('R4001',4003,110,'posted','2026-09-02'),
('R4002',4004,120,'posted','2026-09-03'),
('R4003',4005,130,'posted','2026-09-04'),
('R4004',4006,140,'posted','2026-09-05'),
('R4005',4007,150,'posted','2026-09-06'),
('PART-4000',4003,200,'posted','2026-09-01'),
('OVER-4000',4005,9999,'posted','2026-09-01'),
('ORPH-4000',4999,250,'posted','2026-09-01'),
('END-4000',4006,800,'posted','2026-10-01'),
('R4001',4003,110,'posted','2026-09-02'),
('R4001',4003,110,'posted','2026-09-02'),
('R4002',4004,121,'posted','2026-09-03');

Your work

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SQL query

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Show the reference answer

SELECT refund_id FROM refunds GROUP BY refund_id HAVING MIN(order_id)<>MAX(order_id) OR MIN(amount)<>MAX(amount) OR MIN(status)<>MAX(status) OR MIN(booked_at)<>MAX(booked_at) ORDER BY refund_id

COUNT(*) > 1 confuses harmless repeats with conflicting facts. Compare the values within each key.