sql - 将列中的数组与另一个查询的计数结果相乘

Question

我是 Postgres 和数据库的新手，所以很抱歉。我运行了一个查询，以从一张表中获取每所学校的学生人数。现在我有下表:

school_probs:

school_code(PK bigint)  schoolName(text)    probs(numeric)
1                       CAA             {0.05,0.08,0.18,0.3,0.11,0.28}
2                       CAS             {0.06,0.1,0.295,0.36,0.12,0.065}
3                       CBA             {0.05,0.11,0.35,0.32,0.12,0.05}
4                       CL              {0.07,0.09,0.24,0.4,0.06,0.09}

我将如何将每所学校的计数与概率列中的每个数字相乘。例如:我们学校的学生总数为“CAA” 如果它是 198，那么概率分布将是(0.05*198、0.08*198、0.18*198、0.3*198、0.11*198、0.28*198)。根据结果​​，我可以给学生打分。

我获取计数的查询如下:

SELECT simulated_records.school, COUNT(simulated_records.school) as studentCount INTO CountSchool
FROM simulated_records, school_probs
WHERE simulated_records.school = school_probs.school
GROUP BY simulated_records.school;

Answer 1

要将数组的元素与常量相乘，您需要再次取消嵌套、相乘和聚合。一些注意事项等待。考虑:

• PostgreSQL unnest() with element number

最好使用 ARRAY constructor .

也就是说，对您未公开的表格设计做出一些假设，我也会简化计数:

• Aggregate a single column in query with many columns

到达:

SELECT *, ARRAY(SELECT unnest(p.probs) * r.student_ct) AS scaled_probs
FROM   school_probs p
LEFT   JOIN  (
   SELECT school, COUNT(*)::int AS student_ct
   FROM   simulated_records
   GROUP  BY 1
   ) r USING (school);

或者，将 NULL 数组表示为 NULL 数组:

SELECT *
FROM   school_probs p
LEFT   JOIN  (
   SELECT school, COUNT(*)::int AS student_ct
   FROM   simulated_records
   GROUP  BY 1
   ) r USING (school)
LEFT   JOIN LATERAL (
   SELECT ARRAY(SELECT unnest(p.probs) * r.student_ct) AS scaled_probs
   ) p1 ON p.probs IS NOT NULL;

db<> fiddle here

我建议仅针对 Postgres 10 或更高版本在 SELECT 列表中使用设置返回函数的简单形式，因为:

• What is the expected behaviour for multiple set-returning functions in select clause?