MySQL：如何多次连接同一张表？

Question

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_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。

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

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

Answer 1

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_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。