Set operations & all-item matches

UNION versus UNION ALL

SELECT customer_id FROM online_customers
UNION
SELECT customer_id FROM store_customers;

These comparisons apply to the entire selected row. Inputs need the same number of columns with compatible types. Use UNION ALL when every input event should survive; use UNION when set semantics are intended.

Bought both A and B

Assume purchases(customer_id, product_id).

SELECT customer_id
FROM purchases
WHERE product_id IN ('A', 'B')
GROUP BY customer_id
HAVING COUNT(DISTINCT product_id) = 2;

Repeated purchases of A do not count as buying B.

Bought every required product

Assume required_products(product_id) contains a unique set of required IDs.

SELECT c.customer_id
FROM customers c
WHERE NOT EXISTS (
    SELECT 1
    FROM required_products r
    WHERE NOT EXISTS (
        SELECT 1
        FROM purchases p
        WHERE p.customer_id = c.customer_id
          AND p.product_id = r.product_id
    )
);

Read this as: there is no required product that the customer has failed to purchase. If the required set is empty, every customer qualifies. If that is not the intended rule, add an existence check on required_products.