Filtering joins

Use WHERE together with JOIN.

Join filtering

You filter a JOIN result with WHERE. The JOIN runs first, then WHERE narrows the result.

SELECT o.id, c.name
FROM orders o
INNER JOIN customers c ON o.customer_id = c.id
WHERE c.city = 'İstanbul';

This query returns orders from Istanbul customers.

o.id, c.name is selected from orders o. customers c is joined on o.customer_id = c.id, keeping only matching rows. WHERE keeps only the rows where c.city = 'İstanbul' holds.

Join filtering — details

The join condition goes in ON, the row filter in WHERE. Mixing them can slow the query.

For example, ON o.customer_id = c.id links the tables, while WHERE c.city = 'İstanbul' limits the result to customers in Istanbul.

Exercise Join filtering

Filter the JOIN result where city = 'İstanbul'.

Show example solution

SELECT o.id, c.name
FROM orders o
INNER JOIN customers c ON o.customer_id = c.id
WHERE c.city = 'İstanbul';

Test yourself

Add the filter

SELECT o.id, c.name
FROM orders o
INNER JOIN customers c ON o.customer_id = c.id
____ c.city = 'İstanbul';

Show answer

SELECT o.id, c.name
FROM orders o
INNER JOIN customers c ON o.customer_id = c.id
WHERE c.city = 'İstanbul';

Filtering joins: match the terms

ON

join
WHERE

row filter
HAVING

group filter
JOIN

table
Where does the row filter go?
- FROM
- ON
- SELECT
- WHERE
Show answer

WHERE — The row filter goes in WHERE.

Next lesson: Summary - INNER JOIN

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