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Two Sigma·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say
May 2026

Summary

Two Sigma coding round, one question the whole time. Build a mini in-memory relational database that handles create, insert, and select with a where clause. Took me longer than I'd like to admit to get the predicate parsing right.

Questions Asked (1)

Q1

Implement a simple in-memory relational database that processes tokenized commands: create table, insert into, and select from with a two-condition AND where clause. Return matching rows for each select.

Algorithms & Data StructuresSystem DesignData Modeling
Author's notes

The create and insert parts were fine, basically just a dict of lists.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the command syntax and constraints, then design a simple schema representation using hash maps for tables and rows. Implement a parser that tokenizes commands and dispatches to handlers for create, insert, and select, with select filtering rows by evaluating the two-condition AND clause. Focus on correctness and clean code, and discuss potential extensions like indexing or persistence if time allows.

Pro tip: Demonstrate awareness of real-world database internals by mentioning how your in-memory design could be extended to support indexing or transactions, showing you think beyond the basic requirements.

1. Clarify requirements and assumptions

Ask about command syntax, data types, and expected scale to avoid ambiguity. Confirm whether conditions support only equality or also other operators, and how rows are returned.

2. Design data structures

Choose a schema representation: e.g., a map from table names to table objects, each containing column definitions and a list of rows (each row a map from column to value). Consider using a simple list for rows to keep insertion order.

3. Implement command parsing

Tokenize input strings (e.g., split by spaces, handle quoted strings) and parse into command type and arguments. Use a dispatch mechanism (switch or map) to route to appropriate handlers.

4. Implement command handlers

For create table: validate and store schema. For insert: validate against schema and append row. For select: iterate rows, evaluate the two conditions with AND, and collect matching rows.

5. Test and discuss edge cases

Walk through examples, including empty tables, non-matching conditions, and invalid commands. Mention error handling and potential optimizations like indexing.

Key Points to Mention

  • Choice of data structures (e.g., hash maps for tables and rows) and their trade-offs
  • Tokenization and parsing strategy, including handling of quoted strings or special characters
  • Condition evaluation logic for the two-condition AND clause
  • Error handling for invalid commands or schema mismatches
  • Time and space complexity of operations (insert, select)
  • Potential extensions: indexing, persistence, transactions, or support for more complex queries

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