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Structure temporary query sets with WITH CTEs, and perform operations using UNION, EXCEPT, INTERSECT.
Writing efficient SQL queries requires understanding the relational algebra engine underlying SQL databases. Unlike imperative code where execution flows line-by-line, SQL is declarative: you specify what data you need, and the relational query optimizer determines how to retrieve it using indexes and memory join buffers.
When processing a query, SQL engines execute clauses in this exact order: FROM & JOIN → WHERE → GROUP BY → HAVING → SELECT → DISTINCT → ORDER BY → LIMIT. Knowing this sequence prevents attempting to filter aggregated values in a WHERE clause!
A classic interview trap is writing non-SARGable predicates (like WHERE YEAR(order_date) = 2026) which force full table scans. Always compare un-wrapped indexed columns (like WHERE order_date >= '2026-01-01') to enable B-Tree index range scans!
SQL engines evaluate clauses in a strict logical order: FROM & JOIN -> WHERE -> GROUP BY -> HAVING -> SELECT -> DISTINCT -> ORDER BY -> LIMIT. Understanding this order prevents scoping errors when referencing column aliases.
RDBMS query planners build execution graphs (using cost estimation algorithms) to pick between B-Tree Index Scans, Sequential Scans, Hash Joins, and Nested Loops. Writing SARGable predicates ensures indexes are fully utilized.
Top tech companies like Meta, Amazon, and Google test CTEs & Set Operators by asking candidates to write queries against real e-commerce or HR schemas, optimize high-latency queries, and handle NULL edge cases accurately.