[R2 SQL] Document JOINs, subqueries, and multi-table queries

[Marcinthecloud]

• Updates SQL reference with JOIN clause, subquery syntax, and multi-table CTE examples
• Removes single-table restrictions from limitations page and adds join best practices
• Replaces "unsupported JOIN/subquery" troubleshooting with join performance guidance
• Adds changelog entry

Documentation checklist

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• The change adheres to the documentation style guide.

[github-actions]

This pull request requires reviews from CODEOWNERS as it changes files that match the following patterns:

Pattern	Owners
/src/content/changelog/	@cloudflare/pm-changelogs, @cloudflare/product-owners
/src/content/docs/r2-sql/	@Marcinthecloud, @netgusto, @sesteves, @sejoker, @laplab, @jonesphillip, @elithrar, @jonathanc-n, @cloudflare/product-owners

[Marcinthecloud]

/bonk review this draft pr and ensure:

changes and examples are consistent, clear, and concise
no broken links anywhere
fix trivial issues and make suggestions on the PR
please don't change anything in files outside of this PR this time

[ask-bonk]

Posted 2 inline suggestions.

Issues

• MEDIUM src/content/docs/r2-sql/reference/limitations-best-practices.mdx:38 — The "Multi-table CTEs" row duplicates the CTEs entry directly above it. Remove it.
• LOW src/content/docs/r2-sql/reference/limitations-best-practices.mdx:35 — "Semi-join" and "anti-join" should be lowercase to match the other Notes cells, which are sentence fragments.

Out-of-scope note

src/content/docs/r2-sql/troubleshooting.mdx:20 — The expected exactly 1 table in FROM clause error message may be misleading now that JOINs are supported. Consider verifying whether this error still occurs or updating the description in a follow-up.

[ask-bonk]

Review posted on PR #30826.

Summary of changes
The PR documents newly supported multi-table query capabilities in R2 SQL across 4 files:

• Changelog entry announcing JOINs, subqueries, and multi-table CTEs with examples
• SQL reference adds full JOIN clause documentation, subquery syntax (IN/NOT IN, EXISTS/NOT EXISTS, scalar, derived tables), multi-table CTE examples, and self-joins
• Limitations and best practices removes single-table restrictions and adds join performance guidance
• Troubleshooting replaces unsupported JOIN/subquery errors with performance optimization guidance

Actions taken

• Added the documentation label to the PR
• Posted a review with 2 inline suggestions:
  1. MEDIUM: Remove the redundant "Multi-table CTEs" row in the quick reference table since the "CTEs" row already covers multi-table capabilities
  2. LOW: Lowercase "semi-join" and "anti-join" in the Notes column to match the sentence-fragment style of other cells
• Flagged an out-of-scope pre-existing issue: the expected exactly 1 table in FROM clause error message in the troubleshooting page may be outdated now that JOINs are supported

No build-breaking issues (unescaped MDX, invalid frontmatter, broken links) were found.

github run

[github-actions]

Preview URL: https://ee09e840.preview.developers.cloudflare.com
Preview Branch URL: https://r2sql-joins.preview.developers.cloudflare.com

Files with changes (up to 15)

Original Link	Updated Link
https://developers.cloudflare.com/r2-sql/sql-reference/	https://r2sql-joins.preview.developers.cloudflare.com/r2-sql/sql-reference/
https://developers.cloudflare.com/r2-sql/troubleshooting/	https://r2sql-joins.preview.developers.cloudflare.com/r2-sql/troubleshooting/
https://developers.cloudflare.com/changelog/post/2026-05-14-joins-subqueries-multi-table-queries/	https://r2sql-joins.preview.developers.cloudflare.com/changelog/post/2026-05-14-joins-subqueries-multi-table-queries/
https://developers.cloudflare.com/r2-sql/reference/limitations-best-practices/	https://r2sql-joins.preview.developers.cloudflare.com/r2-sql/reference/limitations-best-practices/