Sql 显示组计数为1的其他列

假设我有一个表

Students

，只有两列

LastName

和

FirstName

。我知道我可以通过以下方式获得所有只有1名的

LastName

s：

select LastName from Students group by LastName having count(*) = 1

---

但是如果我还想显示这些行的名字呢？

您可以使用相关子查询进行筛选：

select s.* 
from students s
where (select count(*) from students s1 where s1.LastName = s.LastName) = 1

或者，如果您有一个主列，您可以使用

不存在
：

select s.*
from student s
where not exists (
    select 1 from students s1 where s1.LastName = s.LastName and s1.id <> s.id
)

你也可以这样做

Select * from Students where lastname in (Select lastname from Students group by lastname having count(*) =1)

只需将其添加到
选择中
：

select s.LastName, min(s.firstname)
from Students s
group by s.LastName
having count(*) = 1;

如果只有一行匹配，则
min（）
返回该行上的值。
您需要使用任何可以返回所需值的聚合函数来获取该值。Min，Max，first_value（），ir string_agg（）在其他用例中，您可能希望所有名称都使用lastname

select LastName, 
first_value(firstname) over (order by firstname) firstname,
Min(firstname)
from Students 
group by LastName having count(*) = 1

我自己找到了另一个：

with LastNames as (
  select LastName from Students group by LastName having count(*) = 1
)
select LastName, FirstName from Students
  where LastName in (select LastName from LastNames)

您是否尝试过在select语句中包含firstname，如下所示：select LastName，firstname from STUDIERS group by LastName having count（*）=1@LemuelBotha，您不能包含不是聚合函数或不在group by COLUMNS中的列。是的，您是对的
with LastNames as (
  select LastName from Students group by LastName having count(*) = 1
)
select LastName, FirstName from Students
  where LastName in (select LastName from LastNames)