mysql - MySQL：如何多次加入同一张表？

Question

我有两张桌子ticket和attr. 表ticket有ticked_id字段和其他几个字段。表attr有 3 个字段：

ticket_id - numeric
attr_type - numeric
attr_val - string

attr_type是值的固定枚举。例如，它可以1是2或3。

我需要进行查询，其结果将是 4 列：

ticket_id,attr_val对于attr_type=1,attr_val对于attr_type=2,attr_val对于attr_type=3

如果表中没有对应的值，则应attr_type在attr对应列中显示 NULL 值。

例子：

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 次左连接表，但无法弄清楚如何安排输出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。

Answer 2

虽然您可以使用别名左连接，但在这种情况下，您还可以使用分组和条件表达式的组合：

select t.ticket_id,
       max(case when a.attr_type=1 then a.attr_val end) attr_val1,
       max(case when a.attr_type=2 then a.attr_val end) attr_val2,
       max(case when a.attr_type=3 then a.attr_val end) attr_val3
from ticket t
left join attr a on t.ticket_id = a.ticket_id
group by t.ticket_id

Answer 3

您使用表别名

例如：

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 (select * from attr where attr_type=1) a1 on ticket.ticket_id=a1.ticket_id
    left join (select * from attr where attr_type=2) a2 on ticket.ticket_id=a2.ticket_id
    left join (select * from attr where attr_type=3) a3 on ticket.ticket_id=a3.ticket_id