SQL Joins

Status: draft · Confidence: low (0.43) · Basis: verified_sources

Quality notes: generic_source_homepage, no_verified_sources, partial_source_verification

## TL;DR

SQL JOINs combine rows from two or more tables based on related columns. Types: INNER JOIN (matching rows only), LEFT JOIN (all left rows + matching right), RIGHT JOIN, FULL OUTER JOIN, CROSS JOIN (Cartesian product). Self-join: join table to itself. JOINs are the most powerful and frequently misused SQL feature.

## Core Explanation

INNER JOIN: `SELECT * FROM orders JOIN customers ON orders.cust_id = customers.id` — only rows with matches in both. LEFT JOIN: keeps all left rows, NULL for missing right — used for 'show all customers and their orders if any'. JOIN vs. subquery: EXPLAIN ANALYZE to compare — often equivalent, sometimes one is better. JOIN ordering matters: the query optimizer reorders, but hint with proper statistics.

## Further Reading

- [PostgreSQL Documentation — Joins](undefined)

## Related Articles

- [Text-to-SQL: Natural Language Database Querying with Large Language Models](../../ai/text-to-sql.md)
- [SQL Injection](../sql-injection.md)
- [SQL Query Optimization: Join Algorithms, Cost Models, and Index Selection](../sql-query-optimization-join-algorithms-cost-models-and-index-selection.md)