Start by clarifying the schema and the exact requirement (orphaned employees). Then present a correct SQL query using either NOT EXISTS or LEFT JOIN ... IS NULL, and briefly compare the two approaches in terms of performance and readability. Finally, mention edge cases like NULL department IDs and indexing.
Pro tip: In interviews, explicitly state your assumption about NULL department IDs and how your query handles them; this shows attention to data quality and prevents subtle bugs. Also, mention that NOT EXISTS is often more efficient than NOT IN when NULLs are present.
Confirm the columns of EMPLOYEE and DEPARTMENT tables, especially the join key (e.g., dept_id) and whether NULLs are allowed. Restate the goal: find employees whose department ID has no matching record in DEPARTMENT.
Select an approach: NOT EXISTS, LEFT JOIN with IS NULL, or NOT IN (with caution). Explain why you prefer one, e.g., NOT EXISTS handles NULLs safely and often performs well.
Write the query clearly, using aliases and proper formatting. For example: SELECT e.* FROM EMPLOYEE e WHERE NOT EXISTS (SELECT 1 FROM DEPARTMENT d WHERE d.dept_id = e.dept_id);
Mention indexing on the join columns, and compare NOT EXISTS vs LEFT JOIN vs NOT IN in terms of execution plan and NULL handling. Note that NOT IN can return unexpected results if the subquery returns NULL.
Consider employees with NULL department IDs (they won't match any department, so they should be included if the requirement is 'does not exist'). Suggest testing with sample data or explaining how to verify correctness.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.