Question

I get customer ids for customers with sites that have appointments in 2011

SELECT customers.id
FROM customers 
INNER JOIN sites ON customers.id = sites.customer_id
INNER JOIN appointments ON appointments.site_id = sites.id
WHERE YEAR(appointments.day) = 2011   

I’m looking to get customers who don’t have any appointments in 2011. Confused on how to go about it.

Answer 1

A couple of ways:

SELECT
    C.id
FROM
    Customers C
WHERE
    NOT EXISTS (
        SELECT *
        FROM
            Sites S
        INNER JOIN Appointments A ON
            A.site_id = S.id AND
            YEAR(A.day) = 2011
        WHERE
            S.customer_id = C.id
    )

or:

SELECT DISTINCT
    C.id
FROM
    Customers C
INNER JOIN Sites S ON
    S.customer_id = C.id
LEFT OUTER JOIN Appointments A ON
    A.site_id = S.id AND
    YEAR(A.day) = 2011
WHERE
    A.id IS NULL