进行“分组依据"会导致多列 [英] Making `group by` result in multiple columns

问题描述

我有一个( Postgres )查询，该查询具有通常的 group by 子句:

I have a (Postgres) query with a usual group by clause:

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 的值为 low ， medium 和 high ，所有这些结果均位于行格式，例如:

The column b.irt_tlevel has values low, medium and high, all these results are in a row-format, for example:

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

我希望结果采用以下格式:

I'd like my results to be in the following format:

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

任何帮助将不胜感激.

推荐答案

如果已知irt_tlevel列中的不同值的数目是固定的，则可以使用条件聚合.

You can use conditional aggregation if it is known that the number of distinct values in irt_tlevel column are fixed.

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

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