MySQL：如何多次连接同一张表？

您需要使用多个

LEFT JOINs

SELECt     ticket.ticket_id,      a1.attr_val AS attr_val1,    a2.attr_val AS attr_val2,    a3.attr_val AS attr_val3FROM 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

这是一个示例：SQL Fiddle。