Mysql 使用 if 语句聚合数据

Question

我试图解决的问题的简化示例：

编辑 - 使用 PK

sqlfiddle: http://sqlfiddle.com/#!2/7be5d/1

---

我有两个table：

People
--------
| Name |
--------
| Mike |
| Jim  |
| Fred |
--------


HasBanana
------------------------
| Name  | Has_a_banana |
------------------------
| Mike  | N            |
| Mike  | N            |
| Mike  | N            |
| Mike  | N            |
| Mike  | Y            |
| Mike  | N            |
| Jim   | N            |
| Jim   | N            |
| Jim   | Y            |
| Jim   | N            |
| Jim   | N            |
| Fred  | N            |
| Fred  | N            |
| Fred  | N            |
| Fred  | N            |
| Fred  | N            |
| Fred  | N            |
------------------------

还有一个查询：

select * from People
left join HasBanana on HasBanana.name = People.name;

这会生成与上面的 HasBanana table 完全相同的 table。我想要生成的是 table，按 People.name 分组，如果此人曾被标记为有香蕉，则显示 Y。

所以像这样：

HadBanana
------------------------
| Name  | Had_a_banana |
------------------------
| Mike  | Y            |
| Jim   | Y            |
| Fred  | N            |
------------------------

我知道我可以使用以下内容按名称分组

group by people.name;

但是如果 has_a_banana 曾经是 Y，则在执行每个名称的逻辑时遇到问题，然后将 had_a_banana 设置为 Y。

Answer 1

select 
   p.name
   , case when x.name is null then 'N' else 'Y' end as had_a_banana
from people p
left outer join
(
  select 
     name 
  from bananas
  where has_a_banana = 'Y'
  group by name
) x
on x.name = p.name
group by p.name, x.name

我已经使用您的 SQLFiddle link 进行了检查，尽管您的 Fiddle 数据与您的问题不符，所以我得到：

Fred  N
Jim   Y
Mike  N

（这似乎与您的 Fiddle 数据匹配）。

Answer 2

</p> <pre><code>select people.id, people.name, case a.has_a_banana when 'Y' then 'Y' else 'N' end as has_a_banana from people left join (select people.id, people.name, bananas.has_a_banana from people left join bananas on bananas.name = people.name where has_a_banana = 'Y' group by people.name) as a on a.name = people.name

应该让你得到你想要的。

Answer 3

替代方案（有点丑陋的解决方案）：

select people.name, max(has_a_banana) from people
left join bananas on bananas.name = people.name
group by people.name;

Answer 4

w=14=WILL.y.:w=12=w w=10=sh w=14=shw=11=shw=12=sh

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