CTEs name intermediate relations and can make complex logic readable. Optimization and materialization behavior depend on the database and query.
Before you start
You should understand tables, keys, joins and SELECT queries. Write down a tiny dataset before reasoning about a query. SQL examples use PostgreSQL-style syntax where relevant; planner behavior, locking and available features must be checked for the database and version used in an application.
The practical goal is to reason through this situation: Separate eligible orders from customer totals to make result grain visible. Read the walkthrough first, then try the interview exercise before opening its answer. The important part is explaining the decision and its consequences, rather than remembering a definition alone.
Step-by-step walkthrough
Step 1: Name intermediate grain
Give meaningful names to eligible orders and customer totals.
Step 2: Preserve semantics
A rewrite must return the same rows before performance is evaluated.
Step 3: Inspect actual planning
Materialization and inlining depend on engine, version and query structure.
Worked scenario
Separate eligible orders from customer totals to make result grain visible.
A CTE isolates eligible orders so reviewers can see filters before aggregation. That readability benefit is independent of whether it is materialized. Treating every CTE as a speed optimization can mislead; compare the plan and result under representative data and the production database version.
Common mistake
Assuming every CTE is always materialized or always faster is unreliable.
Verify the behavior
Compare original and rewritten results, then plans and resource usage.
Interview exercise
Evaluate a rewritten query.
Answer and reasoning
Compare plans and results on representative data, preserving semantics before judging readability or performance.
Continue learning
Compare the scenario with the SQL and PostgreSQL interview questions and test your understanding with the SQL and PostgreSQL MCQs. For terminology and implementation details, consult the reference material.