SQL · Basics & Joins

ON vs WHERE in a JOIN

Easysql-07

onwhereouter join

Question

What is the difference between ON and WHERE in a JOIN?

ON specifies how rows are matched.

WHERE filters the result after the join logically occurs.

Example:

SELECT *
FROM customers c
LEFT JOIN orders o
    ON c.id = o.customer_id
WHERE o.amount > 100;

That WHERE can drop customers who have no matching orders (the LEFT JOIN starts acting like an INNER JOIN).

Compare:

SELECT *
FROM customers c
LEFT JOIN orders o
    ON c.id = o.customer_id
   AND o.amount > 100;

Now customers without qualifying orders can still appear.

Gotcha:

Moving conditions between ON and WHERE is not always the same, especially with LEFT, RIGHT, or FULL OUTER JOIN.

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