MySQL使用联接对多个表中的行进行计数

Question

我有3个MySQL表：

**locations**
id      location_name
1       London
2       New York
3       Washington

**bookings**
id      voucher_code    location_id
1       DISC100         1
2       NULL            NULL
3       DISC200         1
4       DISC500         2
5       DISC700         1

**vouchers**
id      voucher_code    net_value
1       DISC100         155.00
2       DISC200         135.00
2       DISC500         155.00
2       DISC700         155.00

我正在尝试显示每个位置的凭单净值计数。 因此，例如，您可以看到“伦敦”位置有3张已在bookings表中兑换的凭证，其中2张为155.00值的凭证，一张为135.00。 我需要编写一个MYSQL查询以将这些计数从数据库中取出。

因此，以上述表格为例，我的SQL查询将导致以下结果：

Location            netValue155     netVaue135
London              2               1
New York            1               0
Washington          0               0

以下是我到目前为止所写的内容：

select 
    l.location_name,
    '' as date_from, 
    '' as date_to,
    v.netValue155
from locations l 
left join (select voucher_code, count(distinct id) as netValue155 from vouchers where net_value='155.00' group by id) as v

Answer 1

select a.location_name, sum(netvalue155), sum(netvalue135) from
(select l.id ,l.location_name,
case when v.net_value = 155 then 1 else 0 end as netvalue155,
case when v.net_value = 135 then 1 else 0 end as netvalue135
from locations l left join bookings b
on l.id = b.location_id
left join vouchers v
on  b.voucher_code = v.voucher_code) a
right join locations l on l.id = a.id
group by a.location_name

[sql小提琴] http://sqlfiddle.com/#!9/daefd/19

Answer 2

您应该执行以下操作：

SELECT 
    l.location_name AS Location,
    SUM(IF(v.net_value=155, 1, 0)) AS netValue155,
    SUM(IF(v.net_value=135, 1, 0)) AS netVaue135
FROM
    locations AS l
LEFT JOIN 
    bookings AS b ON (l.id = b.location_id)
LEFT JOIN 
    vouchers AS v ON (v.voucher_code = b.voucher_code)
GROUP BY 
    l.id