INNER JOIN

Matching rows only

SELECT o.order_id, c.name
FROM orders o
INNER JOIN customers c ON c.customer_id = o.customer_id;

INNER is the default, so plain JOIN means the same thing. Only rows with a match on both sides appear.

What disappears

Two categories vanish silently:

• Customers with no orders
• Orders whose customer_id matches no customer (orphans, often from bad data)

Neither produces a warning. A report that quietly omits customers who have not ordered is a classic inner-join defect.

Checking for loss

Compare counts:

SELECT COUNT(*) FROM orders;                        -- 1000
SELECT COUNT(*) FROM orders o
JOIN customers c ON c.customer_id = o.customer_id;  -- 987

Thirteen orders reference a customer that does not exist. Worth knowing before you publish a revenue figure.

Multiple tables

FROM orders o
JOIN customers c ON c.customer_id = o.customer_id
JOIN regions   r ON r.region_id   = c.region_id

Joins evaluate left to right, each operating on the accumulated result.