Write SQL to rank advertisers and profitability

Q: How do I practice SQL interview questions?

PracHub provides an interactive SQL console where you can write and test queries against real database schemas. Get instant feedback and compare your solution with the expected output.

Q: What difficulty level is this coding question?

This is a Medium difficulty Data Manipulation (SQL/Python) question, commonly asked during Technical Screen rounds at Instacart.

Q: What role is this question designed for?

This question is commonly asked for Data Scientist candidates at Instacart during technical interviews.

Question

You are given two denormalized tables. Use ANSI SQL (CTEs allowed) and UNPIVOT (or an equivalent UNION ALL approach if UNPIVOT is unavailable).

Table: advertiser_spend
Columns: advertiser_id INT, advertiser_name VARCHAR, spend_prog_A DECIMAL, spend_prog_B DECIMAL, spend_prog_C DECIMAL
Sample data:
+---------------+----------------+--------------+--------------+--------------+
| advertiser_id | advertiser_name| spend_prog_A | spend_prog_B | spend_prog_C |
+---------------+----------------+--------------+--------------+--------------+
| 1 | AlphaCo | 1000 | 500 | NULL |
| 2 | BetaInc | 200 | 1200 | 300 |
| 3 | GammaLLC | 0 | 0 | 0 |
| 4 | DeltaAds | 700 | 800 | 900 |
+---------------+----------------+--------------+--------------+--------------+

Table: program_financials
Columns: program_code CHAR(1), revenue DECIMAL, cost DECIMAL
Sample data:
+--------------+---------+------+
| program_code | revenue | cost |
+--------------+---------+------+
| A | 5000 | 3500 |
| B | 3000 | 2800 |
| C | 2000 | 500 |
+--------------+---------+------+

Mapping: spend_prog_A->'A', spend_prog_B->'B', spend_prog_C->'C'.

Task:
1) Unpivot advertiser_spend into a tall table (advertiser_id, advertiser_name, program_code, spend). Treat NULL spend as 0.
2) Return the single top-spending advertiser overall (exclude advertisers with total_spend = 0). Output columns: advertiser_name, total_spend.
3) Return the program with the highest net_profit = revenue - cost and, separately, the program with the highest profit_margin = CASE WHEN revenue>0 THEN (revenue - cost)/revenue ELSE NULL END. If both metrics select the same program, return only one row; otherwise return two rows with a column reason IN ('highest_net_profit','highest_margin'). Break ties by higher revenue, then alphabetical program_code.
4) Your solution must use the unpivoted rows to compute advertiser totals and a JOIN to program_financials only where needed. Clearly show the final SELECT(s).

PracHub · Accepted Answer

This question evaluates a candidate's ability to perform ANSI SQL data transformations including unpivoting denormalized tables, aggregations, joins, and financial metric calculations such as net profit and profit margin while handling nulls and tie-breaking rules.

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