即使计数值为0 mysql也要选择行

Question

select c.id, count(*) as 'Transactions' from voucher v 
right join consumer c on v.consumer_id = c.id
where v.voucher_state in ('REDEEMED', 'EXPIRED') and c.country = 'INDIA'
group by c.id;

预期产量： -

ID交易中获得

3     0

4     0

6     3

7     9

8     4

9     0

目前的输出： -

ID交易中获得

6     3

7     9

8     4

如何选择count = 0的行？谢谢。

Answer 1

你必须使用LEFT JOIN，在左侧放置consumer表并移动

 v.voucher_state in ('REDEEMED', 'EXPIRED'):

到ON条款：

select c.id, count(v.consumer_id) as 'Transactions' 
from consumer c
left join voucher v  on v.consumer_id = c.id on v.voucher_state in ('REDEEMED', 'EXPIRED')
where c.country = 'INDIA'
group by c.id;

上面的查询将返回满足条件的所有customer.id值

customer.country = 'INDIA'

只有在voucher表中具有匹配记录的客户才有

voucher.voucher_state in ('REDEEMED', 'EXPIRED')

将被计算在内。

Answer 2

由于您使用的是RIGHT JOIN，因此左表上的过滤条件必须位于ON子句中。原因是，过滤将在优化器将其与正确的表连接之前发生。 WHERE子句将在最终结果中过滤。

另外，你必须只计算左表中的行（例如COUNT(v.consumer_id))）而不是COUNT(*)，否则，每个c.id总会有一个计数。

select c.id, count(v.consumer_id) as 'Transactions' 
from voucher v 
     right join consumer c 
       on v.consumer_id = c.id 
          and v.voucher_state in ('REDEEMED', 'EXPIRED')
where  c.country = 'INDIA'
group  by c.id;

Answer 3

使用IFNULL

select c.id,IFNULL(COUNT(*), 0) as 'Transactions' from voucher v 
right join consumer c on v.consumer_id = c.id
where v.voucher_state in ('REDEEMED', 'EXPIRED') and c.country = 'INDIA'
group by c.id;