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TikTok·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
Jun 2026

Summary

TikTok Data Engineer interview with a SQL-heavy technical screen. The question was pretty involved and required walking through a query step by step rather than just writing one from scratch.

Questions Asked (1)

Q1

Given two tables and a specific SQL query, explain exactly what the result set looks like: which rows get returned, what each column contains, how joins, filters, aggregations, and window functions shape the output, how NULLs and duplicates are handled, and what the final ordering is.

Data ModelingTechnical Trade-offsAlgorithms & Data Structures
Author's notes

This tripped me up more than I expected.

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AI HintsAI Generated

Suggested Approach

Break the query into its logical clauses (FROM/JOIN, WHERE, GROUP BY, HAVING, SELECT, window functions, ORDER BY) and trace the data step by step. For each step, describe how rows are filtered, combined, grouped, or transformed, and how NULLs and duplicates are handled. Finally, summarize the exact result set: columns, row values, and ordering.

Pro tip: Mention that window functions are evaluated after WHERE, GROUP BY, and HAVING but before ORDER BY, and that they do not collapse rows like GROUP BY does. Also note that NULLs are treated as equal in GROUP BY and DISTINCT but not in comparisons, and that duplicate rows are preserved unless DISTINCT or GROUP BY is used.

1. Parse the query into logical clauses

Identify the order of evaluation: FROM/JOIN, WHERE, GROUP BY, HAVING, SELECT, window functions, ORDER BY, LIMIT. This determines how data flows and transforms.

2. Analyze joins and filters

Determine how tables are combined (INNER, LEFT, etc.) and which rows are removed by WHERE. Explain how NULLs from outer joins affect subsequent steps.

3. Evaluate aggregations and window functions

For GROUP BY, describe how rows collapse into groups and how NULLs are grouped. For window functions, explain partitioning, ordering, and framing, and how they compute values without collapsing rows.

4. Determine the final SELECT and ORDER BY

List the output columns and their values after all transformations. Specify the final ordering, including how NULLs are sorted (e.g., NULLS FIRST/LAST) and tie-breaking.

5. Summarize the result set

State exactly which rows are returned, what each column contains, and how duplicates and NULLs are handled in the final output.

Key Points to Mention

  • Logical order of SQL evaluation: FROM → WHERE → GROUP BY → HAVING → SELECT → window functions → ORDER BY → LIMIT.
  • How different join types (INNER, LEFT, RIGHT, FULL) affect row inclusion and NULL generation.
  • Aggregation with GROUP BY collapses rows; NULLs form their own group; HAVING filters after grouping.
  • Window functions compute across a set of rows without collapsing them; PARTITION BY and ORDER BY define the window; frame clause affects results.
  • NULL handling: comparisons with NULL yield UNKNOWN; NULLs are considered equal for GROUP BY and DISTINCT; ORDER BY can specify NULLS FIRST/LAST.
  • Duplicates: preserved unless DISTINCT or GROUP BY is used; DISTINCT treats NULLs as equal.

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