我有两个桌子ticket和attr。表ticket具有ticked_id字段和其他几个字段。表attr具有3个字段：
ticket
attr
ticked_id
ticket_id - numeric attr_type - numeric attr_val - string
attr_type是固定的值枚举。例如，它可以是1，2或3。
attr_type
1
2
3
我需要进行查询，其结果将是4列：
ticket_id，attr_val对attr_type=1，attr_val对attr_type=2，attr_val对attr_type=3
ticket_id
attr_val
attr_type=1
attr_type=2
attr_type=3
如果不存在用于相应值attr_type在attr表然后NULL值应在相应的列中显示。
例：
ticket ticket_id: 1 ticket_id: 2 ticket_id: 3 attr ticket_id: 1 attr_type: 1 attr_val: Foo ticket_id: 1 attr_type: 2 attr_val: Bar ticket_id: 1 attr_type: 3 attr_val: Egg ticket_id: 2 attr_type: 2 attr_val: Spam
结果应该是：
ticked_id: 1 attr_val1: Foo attr_val2: Bar attr_val3: Egg ticked_id: 2 attr_val1: NULL attr_val2: Spam attr_val3: NULL ticked_id: 3 attr_val1: NULL attr_val2: NULL attr_val3: NULL
我尝试了attr3次左联接表，但无法弄清楚如何排列输出attr_type
您需要使用多个LEFT JOINs：
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
这是一个示例：SQL Fiddle。
