Felt fine at first, wrote the basic LEFT JOIN no problem.
Start by clarifying the schema and the exact requirement: all parent rows with matching child rows, including parents without children. Then write a LEFT JOIN query with proper aliasing, select parent columns and child columns, and handle NULLs on the child side. Finally, extend the pattern to multiple tables by chaining LEFT JOINs and discussing performance considerations.
Pro tip: Mention that you would verify the join keys and indexes, and consider whether a LEFT JOIN with a WHERE clause on the child table could accidentally filter out unmatched parents—a common pitfall. Also, note that for multiple tables, the order of joins can affect readability and performance.
Ask about the table structures, join keys, and whether all parent rows are needed even if no child exists. Confirm the expected output columns and any filtering conditions.
Construct a query selecting from the parent table LEFT JOIN the child table on the foreign key. Use table aliases for clarity and select parent columns plus child columns.
Explain that unmatched child columns will be NULL. If needed, use COALESCE or CASE to provide default values, and ensure any WHERE conditions on child columns are in the ON clause to preserve unmatched parents.
Show how to add more LEFT JOINs for additional related tables, maintaining the parent as the left side. Discuss how to avoid fan-out and ensure correct aggregation if needed.
Mention indexing on join keys, the impact of join order, and alternatives like subqueries or EXISTS if only existence is needed. Highlight readability and maintainability.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.