sql - 进行分组依据会导致多列

我有一个（Postgres）查询，其中包含一个常用的group by子句：

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,a.testscoreid, b.irt_tlevel
order by Year desc, a.testscoreid

列b.irt_tlevel具有值low、medium和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

任何帮助都将不胜感激。

最佳答案

如果已知irt t t level列中不同值的数目是固定的，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

关于sql - 进行分组依据会导致多列，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/34500598/