Relational SQL

Practical Guide to SQL JOINs: Verified Queries with Real Data

Accidental cartesian products and silent row dropping are the two most common errors in SQL analytics. Learn how to test your schemas and verify join results in a sandbox.

1. INNER JOIN vs. LEFT JOIN

An INNER JOIN preserves only rows where keys match in both tables. A LEFT JOIN retains all rows from the primary table, filling missing matches with NULL.

2. Copyable Schema & Verified Query

CREATE TABLE customers (id INT, name TEXT);
CREATE TABLE orders (id INT, customer_id INT, amount REAL);

SELECT c.name, COUNT(o.id) as orders_count, COALESCE(SUM(o.amount), 0.0) as total_spent
FROM customers c
LEFT JOIN orders o ON c.id = o.customer_id
GROUP BY c.id, c.name;

Run Your Query in the Sandbox

Ask English questions grounded in your schema on English-To-SQL:

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