INNER JOIN

1. INNER JOIN

What it does

Returns only the rows where a match exists in both tables. Any row in either table that has no matching row in the other table is completely dropped from the result.

Think of it as the strictest join — it requires both sides to agree.

Visual Diagram

useState diagram

SQL

SELECT e.name, d.dept_name
FROM employees e
INNER JOIN departments d ON e.dept_id = d.dept_id;

Result

name	dept_name
Alice	Engineering
Bob	Marketing

Why Carol and David are excluded

Carol: her dept_id is NULL. In SQL, NULL = 1 is not TRUE — it is UNKNOWN. The ON clause requires TRUE, so Carol never matches any department row.
David: his dept_id = 4, but no row in the departments table has dept_id = 4. No match → excluded.
Finance (dept_id 3): no employee has dept_id = 3. No match → excluded from the result.

When to use INNER JOIN

Use it when you only care about records that have complete data on both sides — e.g., orders with valid customers, employees with valid departments.