Group By 并获取最常出现的不同值

Question

我想按 varchar 列进行分组并找到出现次数最多的外键值。问题是多个 fiModel 可以分配给同一个 TAC（称为 SSN_Number 的 15 位值的前 8 位）。

这是一个简化的模型和带有示例数据的查询：

create table #data(
    SSN_Number varchar(15),
    fiModel int
)
insert into #data
        SELECT '351806038155151',451 UNION ALL SELECT '353797028764243',232 UNION ALL SELECT '353797028764243',438 UNION ALL SELECT '353797028764243',438 UNION ALL SELECT '353797028764243',447 UNION ALL SELECT '358372015611578',318 UNION ALL SELECT '352045039834626',279 UNION ALL SELECT '352045031234567',279 UNION ALL SELECT '351806035647381',451 UNION ALL SELECT '352045037654321',207

--- following query returns all records(10)
select * from #data Order By SSN_Number

--- following query gives the distinct TAC's+fiModel, but TACs can repeat (9)
select substring(ssn_number,1,8)as TAC,fiModel,count(*) from #data
group by substring(ssn_number,1,8),fiModel
Order By substring(ssn_number,1,8),fiModel

--- following query gives the correct(distinct) TAC's (4), 
--- but i need the fiModel that occurs most often with the assigned TAC
--- if the number is the same, it doesn't matter what to take
select substring(ssn_number,1,8)as TAC,count(*) from #data
group by substring(ssn_number,1,8)
Order By substring(ssn_number,1,8)

drop table #data

所以这是期望的结果：

TAC         fiModel
35180603    451
35204503    279
35379702    438
35837201    318

Answer 1

这应该可以解决问题（CTE 来救援！）：

;with cte as (
    select substring(ssn_number,1,8) as TAC, fiModel, ROW_NUMBER() OVER (PARTITION BY substring(ssn_number,1,8) ORDER BY count(*) desc) as row
    from #data
    group by substring(ssn_number,1,8),fiModel
)
select TAC, fiModel
from cte
where row = 1

作为子查询：

Select TAC,fiModel
from(
    Select substring(ssn_number,1,8)as TAC, fiModel
      ,ROW_NUMBER() OVER (PARTITION BY substring(ssn_number,1,8) ORDER BY count(*) desc) as row
    from #data
    group by substring(ssn_number,1,8),fiModel
)as data
where row=1