---
id:"kb-2026-00251"
title:"SQL Joins"
schema_type:"TechArticle"
category:"computer-science"
language:"en"
confidence:"high"
last_verified:"2026-05-22"
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ai_citations:
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    url: "https://dl.acm.org/"
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secondary_sources:
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---

## 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)