IS NULL on any NOT NULL column of the inner side
The test is on order_id, not on the join key customer_id. order_id is NOT NULL.
task
Customers without orders with LEFT JOIN, testing o.order_id IS NULL.
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The test is on order_id, not on the join key customer_id. order_id is NOT NULL.
Customers without orders with LEFT JOIN, testing o.order_id IS NULL.