Musíte použít více LEFT JOINs
:
SELECT
ticket.ticket_id,
a1.attr_val AS attr_val1,
a2.attr_val AS attr_val2,
a3.attr_val AS attr_val3
FROM ticket
LEFT JOIN attr a1 ON ticket.ticket_id=a1.ticket_id AND a1.attr_type=1
LEFT JOIN attr a2 ON ticket.ticket_id=a2.ticket_id AND a2.attr_type=2
LEFT JOIN attr a3 ON ticket.ticket_id=a3.ticket_id AND a3.attr_type=3
Zde je příklad:SQL Fiddle .