The Data Engineering Interview Answer Vault bundles 750+ reviewed answers into 7 focused PDF volumes — SQL, Spark, Python, System Design, Cloud, Behavioral, and Data Modeling. Discuss the tech stacks and responsibilities at Morgan Stanley Create a script to parse and transform a JSON file into a structured CSV.
- If multiple keys have the same value, map that value to a list of the original keys.
- This can be beneficial in many analysis scenarios where extreme values might skew results.
- It can automatically perform sort, total, count, or average of the data in the spreadsheet and display the results in another spreadsheet.
- UNION combines the results of two queries and removes duplicates, which requires an expensive sorting operation.
- Binary search efficiently finds items in sorted lists by repeatedly halving the search range.
It typically includes tools for ingestion (Fivetran), storage (Snowflake), transformation (dbt), and visualization (Tableau). This enables "Partition Pruning," allowing a query to skip entire folders of irrelevant data. It also features built-in compression and metadata, making it much faster and cheaper for large-scale queries. For platform-specific preparation, see our interview guides on Snowflake and Databricks. Check out our data engineering courses for structured, hands-on learning with real-world projects and expert guidance.
The follow-up they askWhy is COUNT(DISTINCT) so much harder to distribute across nodes than SUM? On billions of rows the working tool is APPROX_COUNT_DISTINCT (HyperLogLog, roughly 2% error, constant memory); know where the error is acceptable, e.g. dashboard audience counts yes, billing no. The follow-up they askSame table, now find employees who earn more than their own manager. The follow-up they askHow do NULLs in a GROUP BY behave differently than in a join key? NOT EXISTS with a correlated subquery, LEFT JOIN with WHERE b.key IS NULL, and NOT IN. The follow-up they askWhen would you fix it with a window function instead of pre-aggregation?
Every senior and staff data engineering loop has a round that LeetCode-style prep can't touch — and most candidates walk in having never practiced it once. Syntax highlighting, autocomplete, shortcuts you already know. See how to select a question, write real code in the browser, run it and instantly see if you pass or fail.
Graph and Tree Algorithms for Data Engineers
If you work in data but haven't interviewed at this bar, roughly a month at 2 hours a day. The screen filters you out before anyone sees your Python. Onsites add sessionization, gaps-and-islands, and design follow-ups. You writea SQL query, the same shape a screen would give you. Each stage sits where its skill comes up in the loop, with rough hours attached; compress or stretch them to your interview date.
The follow-up they askDead-letter rate jumped from 0.1% to 4% overnight but the pipeline is green. The follow-up they askWhat does __exit__ returning True do, and when is swallowing the exception correct? @contextmanager
https://uvik.io/ with try/finally around the yield is the five-line version. The follow-up they askThe operation is a payment post.
The Hadoop Distributed File System (HDFS) cluster's safe mode for the NameNode is read-only. The HDFS Balancer is basically a utility for balancing data across an HDFS cluster's storage devices. It lowers network congestion while increasing total system throughput.
Spark doesn't execute transformations immediately; it builds a Directed Acyclic Graph (DAG) of the plan. RDDs (Resilient Distributed Datasets) are low-level collections without a schema. UNION ALL combines results but keeps all rows, making it much faster. OLAP (Online Analytical Processing) is optimized for complex data analysis and heavy read operations over massive datasets (e.g., sales trends).
