INNER JOIN

Syntax

SELECT *
FROM customers c
INNER JOIN orders o
  ON c.customer_id = o.customer_id;

What it does

An INNER JOIN keeps only the rows with matching keys in both tables. It is the most common join when filtering to existing relationships.

Example

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

Common pitfalls

• the join key may be wrong or mismatched
• duplicate rows may appear if the join key is not unique in one or both tables
• downstream filters may hide rows you expected to see

Safety check before shipping

• confirm the join key represents the intended relationship
• inspect row counts before and after the join
• decide whether duplicates are valid or indicate a broken relationship