The FDE SQL Interview: What to Expect and How to Prepare
Forward Deployed Engineers live in the customer's data, so nearly every FDE loop includes a SQL round — and it is more demanding than a basic SELECT/WHERE screen. Expect multi-step transformations, window functions, and questions that reward you for reasoning about data correctness.
What the FDE SQL round tests
The round is less about trivia and more about whether you can produce correct results from realistic, imperfect data. Interviewers watch for a few specific habits.
Grain and join correctness
The single most common mistake is silent fan-out: joining a one-to-many relationship and double-counting a metric without noticing. Strong candidates state the grain of every table — "one row means one order line" — before joining, and sanity-check row counts after each step.
Window functions
ROW_NUMBER, RANK, and SUM/AVG OVER (...) answer "top N per group" and running totals without collapsing rows the way GROUP BY does. This is the single biggest separator in FDE SQL rounds.
CTEs and readability
Real transformations are multi-step. Breaking them into named CTEs shows you can write SQL a customer's team could maintain — which is the job.
NULL handling
NULL never equals anything (use IS NULL), and aggregates silently skip NULLs. A careless NULL is a classic wrong-answer trap.
How to prepare
Practice against a real database, not flashcards — the muscle memory of writing and running queries is what the round tests. Work through GROUP BY with HAVING, multi-table joins, and especially window functions until "top earner per department" and "running total per customer" are automatic.
Rung includes a live in-browser SQL sandbox with 19 problems from easy GROUP BYs to Hard window-function and correlated-subquery questions, each checked against a reference result.
Practice FDE SQL in a live sandbox
Practice FDE SQL in a live sandbox →Frequently asked questions
What SQL is tested in a Forward Deployed Engineer interview?
GROUP BY and HAVING, multi-table joins, subqueries, CTEs, and — most importantly — window functions (ROW_NUMBER, RANK, SUM/AVG OVER). Questions are usually multi-step transformations against a realistic schema rather than single-line trivia.
Do FDE interviews test window functions?
Yes — window functions are one of the strongest separators in the SQL round. Expect "top N per group", running totals, and ranking questions that cannot be answered cleanly with GROUP BY alone.
How do I prepare for a SQL interview quickly?
Practice writing and running real queries, not memorizing syntax. Focus on join grain (to avoid double-counting), window functions, and multi-step CTEs. A live SQL sandbox that checks your answer against a reference result builds the right instincts fastest.