Add lesson 19: CTEs

[exekias]

Adds the second lesson of the new Module 5: Intermediate querying — common table expressions (WITH, readability, chaining).

What's covered

• The readability problem: a gnarly nested subquery rewritten with WITH — name your steps
• Chaining multiple CTEs (filter → aggregate → join back)
• Referencing one CTE more than once in the main query
• A CTE works anywhere a table can appear (e.g. feeding an IN subquery)
• Materialization in brief: Postgres 12+ inlines single-use CTEs; AS MATERIALIZED / AS NOT MATERIALIZED override
• Pointers to data-modifying CTEs (WITH ... DELETE ... RETURNING) and the later recursive-CTE lesson

Seed

A tiny music-streaming dataset: artists, tracks, and plays (25 listens across June 2024), skewed so multi-step CTE chains produce meaningful results.

SELECT-only lesson, so per CONTRIBUTING.md it has no checks.

Note: this PR also bootstraps lessons/05-intermediate-querying/module.yaml, shared byte-for-byte with the sibling conditional-expressions and window-functions PRs so they merge cleanly.

Part of #6

🤖 Generated with Claude Code

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Project	Deployment	Actions	Updated (UTC)
learn-postgres	Ready	Preview	Jun 11, 2026 10:28am