进行“分组依据”会导致多列

如何解决进行“分组依据”会导致多列

如果已知irt_tlevel列中不同值的数量是固定的，则可以使用条件聚合。

select 
extract(year from a.created) as Year,
a.testscoreid, 
sum(case when b.irt_tlevel = 'Low' then 1 else 0 end) as Low,
sum(case when b.irt_tlevel = 'Medium' then 1 else 0 end) as Medium,
sum(case when b.irt_tlevel = 'High' then 1 else 0 end) as High,
count(*) as Questions
from asmt.testscores a 
join asmt.questions b on a.questionid = b.questionid
where a.answered = True
group by Year, a.testscoreid
order by Year desc, a.testscoreid

解决方法

我有一个Postgres带有通常group by子句的（）查询：

select extract(year from a.created) as Year,a.testscoreid,b.irt_tlevel,count(a.*) as Questions
from asmt.testscores a join asmt.questions b
on a.questionid = b.questionid
where a.answered = True
group by Year,b.irt_tlevel
order by Year desc,a.testscoreid

该列b.irt_tlevel具有low，medium和值，high所有这些结果均为行格式，例如：

Year    TestScoreId    Irt_tlevel    Questions
2015    1              Low           2
2015    1              Medium        3
2015    1              High          5

我希望我的结果采用以下格式：

Year    TestScoreId    Low    Medium    High    TotalQuestions
2015    1              2      3         5        10

任何帮助，将不胜感激。