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1. 问题描述

在Hive中（其他类似SQL，比如PostgreSQL可能也存在此问题），当对多张表（3张及以上）进行full join时，会存在每张表的主键都是唯一，但当full join后，会发现主键可能有重复。

2. 问题复现

2.1. 插入数据

with   temp1 as (
select  '1' as id ,'张三' as name
union all 
select  '2' as id ,'李四' as name
union all 
select  '3' as id ,'王五' as name
),
temp2 as (
select  '1' as id ,'深圳' as city
union all 
select  '3' as id ,'北京' as city
union all 
select  '4' as id ,'上海' as city
),
temp3 as (
select  '1' as id ,'5000' as salary
union all 
select  '4' as id ,'10000' as salary
union all 
select  '5' as id ,'12000' as salary
)

2.2. 查询SQL以及问题

selectcoalesce(a.id, b.id, c.id) as id, a.name, b.city, c.salary
from temp1 as afull join temp2 as b
on a.id = b.idfull join temp3 as c
on a.id = c.id

当执行如上查询SQL时，会发现其中 id = 4 的数据有重复，如下图所示：

3. 问题原因

之所以会出现这样的问题，是因为是以a表为主表，但a表中只有 id 为 1、2、3 的数据，但在b表中有id为4，c表中也有id为4，此时full join时，a表会以空值和b表、c表中id为4的数据join，这样关联后的表中就会出现2条id为4的数据；

4. 问题解决

在后续的表full join时，不单单使用第一张表的主键full join（因为是full join，所以肯定会存在第一张表为null，而其他表有值的数据），而使用 coalesce 方法对所有前面已经full join了的主键进行条件关联，如下代码：
方法1：

selectcoalesce(a.id, b.id, c.id) as id, a.name, b.city, c.salary
from temp1 as afull join temp2 as b
on a.id = b.idfull join temp3 as c
on coalesce(a.id, b.id) = c.id

结果如下：

方法2：

select temp.id, temp.name, temp.city, c.salary
from  
(selectcoalesce(a.id, b.id) as id, a.name , b.city 
from temp1 as afull join temp2 as b
on a.id = b.id) temp
full join 
temp3 as c
on temp.id = c.id

方法3：

select 
temp.id 
,temp1.name
,temp2.city
,temp3.salary
from 
(select id 
from  
(select 
id
from  temp1
union all 
select 
id
from  temp2
union all 
select 
id
from  temp3
) tt
group by id
) temp 
left  join temp1  
on temp.id = temp1.id
left  join temp2  
on temp.id = temp2.id
left  join temp3 
on temp.id = temp3.id