All FDE interview questions

Question 1Data and SQL

Write a query that finds each customer's first and most recent order.

A quick check that you're comfortable with grouping, joins and window functions.

Asked for in 46% of live FDE postings
“Data Engineering & Integration: Proficiency in complex SQL, ETL/ELT pipeline design, and experience building system integrations connecting AI solutions to enterprise and telecom data sources.”
Senior Engineer, Forward Deployment · T-Mobile
They scoreAsks what counts firstPicks the right toolProves it on real rows

The answerFour steps

  1. 1

    Clarify the tables and what counts as an order (cancelled ones?)

    Say it like this

    “First I'd ask what counts as an order: do cancelled or refunded ones count, and is created_at in UTC?”

    Why it scores

    Pins down the definition before the syntax. Cancelled orders change the answer.

  2. 2

    Use MIN/MAX with GROUP BY, or ROW_NUMBER() if you need the whole row

    Say it like this

    “If they only need dates, it's one pass: MIN(created_at) and MAX(created_at) with GROUP BY customer_id. If they want the whole row, like the amount of the first order, I'd use ROW_NUMBER() OVER (PARTITION BY customer_id ORDER BY created_at), once ascending and once descending, and keep row 1 of each.”

    Why it scores

    Knows when the simple version is enough and when you need the whole row.

  3. 3

    Handle ties on the same timestamp, and customers with no orders

    Say it like this

    “Two orders can share a timestamp, so I add order_id to the ORDER BY to make it deterministic. Customers with no orders only show up if I start from customers with a LEFT JOIN.”

    Why it scores

    Ties and missing customers are the bugs interviewers wait for.

  4. 4

    Explain how you'd test it on a few known customers

    Say it like this

    “Then I pick three customers to check by hand: one with a single order, one with 40, one whose first order was cancelled.”

    Why it scores

    Tests on cases chosen to break it, not on whatever comes back first.

StrongAll together

About a minute, out loud. The marked parts are the four steps; hover one to find it above.

First I'd ask what counts as an order: do cancelled or refunded ones count, and is created_at in UTC?1 Say we exclude cancelled. If they only need dates, it's one pass: MIN(created_at) and MAX(created_at) with GROUP BY customer_id. If they want the whole row, like the amount of the first order, I'd use ROW_NUMBER() OVER (PARTITION BY customer_id ORDER BY created_at), once ascending and once descending, and keep row 1 of each.2 Two orders can share a timestamp, so I add order_id to the ORDER BY to make it deterministic. Customers with no orders only show up if I start from customers with a LEFT JOIN.3 Then I pick three customers to check by hand: one with a single order, one with 40, one whose first order was cancelled.4

WeakThe answer that fails

Sounds fine. Common. Scores low.

“I'd group by customer_id and take MIN and MAX of order_date. If they need the other columns, I'd join the orders table back on customer_id and that date to get the full row.”

Interviewer's note

Joining back on a date duplicates rows when two orders share it. Never asked what counts as an order, never tested.

PracticeTwo minutes, then they push back

Your turn

Write a query that finds each customer's first and most recent order.

Answer like you're in the room: about two minutes, no notes. Say it out loud, even alone. It feels different from thinking it. Then check your answer against what interviewers score.