SELECT first_appointment.*, next_appointment.*

FROM (

SELECT customer#, MIN(appoinment_date) AS appointment_date

FROM table_name

GROUP BY customer#

HAVING MIN(appointment_date) >= '20140101' AND MIN(appointment_date) < '20140901'

) AS first_appointment

CROSS APPLY (

SELECT TOP (1) next.*

FROM table_name next

WHERE next.customer# = first_appointment.customer# AND

next.appointment_date > DATEADD(DAY, DATEDIFF(DAY, 0, first_appointment.appointment_date) + 30, 0)

ORDER BY next.appointment

) AS next_appointment