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Others·Data Scientist·Technical Phone Screen·Intermediate

Intermediate
May 2026

Summary

Got a SQL question for a Data Scientist role, one problem, conditional aggregation with quarterly breakdowns. Pretty standard warehouse-style question but the edge case around coins with zero transactions tripped me up a bit.

Questions Asked (1)

Q1

Given a coins table and a transactions table, write a SQL query that returns one row per coin with the summed transaction amounts broken out by quarter (Q1 through Q4) for a given year, plus the total transaction count and total amount. Coins with no transactions in that year should still appear with zeros.

Data ModelingProduct Analytics & Metrics
Author's notes

My first instinct was to just filter by year and group by coin, which got me the totals fine.

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AI HintsAI Generated

Suggested Approach

Use a LEFT JOIN from the coins table to the transactions table, filtering transactions to the given year in the join condition. Then use conditional aggregation with CASE statements to sum amounts per quarter, count transactions, and sum total amounts, ensuring coins with no transactions show zeros via COALESCE or COALESCE(SUM(...), 0).

Pro tip: Always put the year filter in the JOIN condition, not the WHERE clause, to preserve coins with no transactions in that year. Also, use COALESCE or IFNULL to handle NULLs from the LEFT JOIN and return zeros as required.

1. Understand the tables and requirements

Identify the columns in coins (e.g., coin_id) and transactions (e.g., coin_id, transaction_date, amount). Clarify that the output should have one row per coin, with quarterly sums, total count, and total amount for a specific year, including coins with no transactions.

2. Choose the join type and filter placement

Use a LEFT JOIN from coins to transactions to keep all coins. Place the year filter in the ON clause of the join to avoid filtering out coins with no transactions in that year.

3. Aggregate with conditional logic

Use CASE statements inside SUM to calculate quarterly amounts: SUM(CASE WHEN QUARTER(transaction_date) = 1 THEN amount ELSE 0 END) AS Q1, etc. Also compute COUNT(transaction_id) for total count and SUM(amount) for total amount.

4. Handle NULLs and group by coin

Wrap aggregate results with COALESCE(..., 0) to convert NULLs to zeros. Group by coin_id (and any other coin attributes if needed).

5. Write and validate the query

Assemble the full SQL query, ensuring correct syntax and aliases. Mentally test with sample data to confirm coins with no transactions appear with zeros.

Key Points to Mention

  • Use LEFT JOIN to include all coins, even those without transactions.
  • Place the year filter in the JOIN condition (ON clause) to avoid excluding coins with no transactions in that year.
  • Use conditional aggregation with CASE statements for quarterly sums.
  • Apply COALESCE or IFNULL to convert NULLs to zeros for coins with no transactions.
  • Include COUNT(transaction_id) for total transaction count and SUM(amount) for total amount.
  • Group by coin_id (and any other non-aggregated columns from the coins table).

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.