SELECT *
FROM tbl_contract co
LEFT JOIN tbl_customer cu ON cu.customer_id = co.customer_id
WHERE cu.marketing_allowed = 1 AND co.marketing_allowed = 1
GROUP BY cu.id
ORDER BY cu.name ASC

SELECT *
FROM tbl_contract co
LEFT JOIN tbl_customer cu ON cu.customer_id = co.customer_id
GROUP BY co.id

tbl_contract:
id: INT(11) PRIMARY
customer_id: INT(11)
marketing_allowed: TINYINT(1)

tbl_customer:
customer_id: INT(11) PRIMARY
marketing_allowed: TINYINT(1)

SELECT (...)
HAVING cu.marketing_allowed = 1

SELECT *
FROM tbl_contract co
INNER JOIN tbl_customer cu ON cu.customer_id = co.customer_id AND cu.marketing_allowed = 1
WHERE cu.marketing_allowed = 1
ORDER BY cu.name ASC

SELECT *
FROM tbl_contract co
LEFT JOIN tbl_customer cu ON cu.customer_id = co.customer_id
WHERE cu.marketing_allowed = 1 AND co.marketing_allowed = 1

SELECT *
FROM tbl_contract co
   LEFT JOIN tbl_customer cu
        ON cu.customer_id = co.customer_id
       AND cu.marketing_allowed = 1
WHERE co.marketing_allowed = 1

SELECT *
FROM tbl_contract co
LEFT JOIN tbl_customer cu ON cu.customer_id = co.customer_id AND cu.marketing_allowed = 1
WHERE co.marketing_allowed = 1

SELECT
    t1.field1,
    t1.field2,
    ...
    (SELECT
        t2.field3
        FROM table2 t2
        WHERE t2.fieldX=t1.fieldX
    ) AS field3,
    (SELECT
        t2.field4
        FROM table2 t2
        WHERE t2.fieldX=t1.fieldX
    ) AS field4,
FROM table1 t1
WHERE t1.fieldZ='valueZ'
ORDER BY t1.sortedField