MySQL-显示两列，其中第一列值在表C中，第二列值不在表C中

假设有两个表：必须有产品和购买产品 我想为每个客户展示他们已经生产的产品 作为

col A

（GROUP_CONCAT）购买，以及他们仍然需要作为

col B

（GROUP_CONCAT）购买哪些产品

预期结果：

|id_b  | buyed  | left       |
|1b    | 1a     | 2a, 3a     |
|2b    |        | 1a, 2a, 3a |

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PS：如果标题错了，我很抱歉，我只是不知道该说什么

左是一个关键词，你不能用它

以下是解决方案：

select B.id_b, COALESCE(R1.buyer, '') buyer, COALESCE(R2.lft, '') lft
from
B, 
(select B.id_b , GROUP_CONCAT(DISTINCT A2.id_a SEPARATOR ', ') as   buyer
from B
left join A A2
on Exists(select * from C where A2.id_a = C.id_a and C.id_b = B.id_b)
group by B.id_b
) R1,
(
select B.id_b , GROUP_CONCAT(DISTINCT A2.id_a SEPARATOR ', ') as  `left` 
from B 
left join A A2 on  
NOT Exists(select * from C where A2.id_a = C.id_a and C.id_b = B.id_b)
group by B.id_b
) R2
where
B.id_b = R1.id_b and B.id_b = R2.id_b

---

这里有一个方法。获得用

集团公司购买的物品很容易，但不购买却不是一件简单的事

select
t2.id_b,
group_concat(t1.id_a) as buyed,
case 
when group_concat(t1.id_a) is null 
then (select group_concat(id_a order by id_a) from tableA) 
else substring_index((select group_concat(id_a order by id_a) from tableA),
concat(group_concat(t1.id_a order by t1.id_a),','),-1)
end as `left`
from tableB t2
left join tableC t3 on t3.id_b = t2.id_b
left join tableA t1 on t1.id_a = t3.id_a
group by t2.id_b

是的，买了很容易，但不是“不买”，这真的很难，谢谢你的解决方案：在我认为有问题后停下来，这是顺序，如果买的列从2a开始，左列将是1a、2a、3a，因为它只是子字符串，你能看看吗？也许你可以在SQL Fiddle上给我演示一下，为了证明这一点谢谢你给我演示，我认为你的解决方案是正确的，谢谢
select
t2.id_b,
group_concat(t1.id_a) as buyed,
case 
when group_concat(t1.id_a) is null 
then (select group_concat(id_a order by id_a) from tableA) 
else substring_index((select group_concat(id_a order by id_a) from tableA),
concat(group_concat(t1.id_a order by t1.id_a),','),-1)
end as `left`
from tableB t2
left join tableC t3 on t3.id_b = t2.id_b
left join tableA t1 on t1.id_a = t3.id_a
group by t2.id_b