与 group by 进行内部连接并查找列中重复值的计数答案

【问题标题】：inner join with group by and finding count of repeated values in a column与 group by 进行内部连接并查找列中重复值的计数
【发布时间】：2021-09-25 17:25:52
【问题描述】：

我正在使用 PostgreSQL 数据库。

我有表job、job_application、interview，它们的结构如下：

工作表：

select * from job;

job_application表

select * from job_application;

采访表：

select * from interview;

我已经准备了一个查询，通过加入上面的表格来获取一些数据，如下所示（当然，我的实际需求略有不同，稍后我会解释）：

SELECT job_description,
       i1.interview_type,
       hired,
       open_positions - hired   AS remaining,
       Count(i1.interview_type) AS current_count
FROM   job
       INNER JOIN job_application j1
               ON job.id = j1.job_id
       INNER JOIN interview i1
               ON j1.id = i1.jobapplication_id
GROUP  BY i1.interview_type,
          job_description,
          hired,
          open_positions;

上述查询将产生如下数据：

但我的实际要求是针对job_description 获取每个interview_type（L1、L2、HR 等...来自interview 表）的计数。例如如下所示：

谁能指导/解释我如何从我已经准备好的查询中获取上述格式的数据？如果您需要任何信息，请告诉我。

【问题讨论】：

标签： sql database postgresql

【解决方案1】：

您可能对 CrossTab (PostgreSQL Crosstab Query) 有一些运气。

但我没有使用它就到达那里：

SELECT 
    job.job_description
    ,job.hired
    ,job.open_positions - job.hired AS remaining
    ,COALESCE(L1.cnt, 0) AS L1
    ,COALESCE(L2.cnt, 0) AS L2
    ,COALESCE(HR.cnt, 0) AS HR
    ,COALESCE(EXECUTIVE.cnt, 0) AS EXECUTIVE
FROM job

LEFT JOIN
(
SELECT
    ja.job_id
    ,COUNT(*) AS cnt
FROM job_application ja

INNER JOIN interview i1
ON ja.id = i1.jobapplication_id
    
WHERE i1.interview_type = 'L1'

GROUP BY
    ja.job_id
) AS L1
ON L1.job_id = job.id

LEFT JOIN
(
SELECT
    ja.job_id
    ,COUNT(*) AS cnt
FROM job_application ja

INNER JOIN interview i1
ON ja.id = i1.jobapplication_id
    
WHERE i1.interview_type = 'L2'

GROUP BY
    ja.job_id
) AS L2
ON L2.job_id = job.id

LEFT JOIN
(
SELECT
    ja.job_id
    ,COUNT(*) AS cnt
FROM job_application ja

INNER JOIN interview i1
ON ja.id = i1.jobapplication_id
    
WHERE i1.interview_type = 'HR'

GROUP BY
    ja.job_id
) AS HR
ON HR.job_id = job.id

LEFT JOIN
(
SELECT
    ja.job_id
    ,COUNT(*) AS cnt
FROM job_application ja

INNER JOIN interview i1
ON ja.id = i1.jobapplication_id
    
WHERE i1.interview_type = 'EXECUTIVE'

GROUP BY
    ja.job_id
) AS EXECUTIVE
ON EXECUTIVE.job_id = job.id;

换句话说，对 Job_application 和面试表的 SQL 查询进行连接，并过滤到每种面试类型（如果有更多的面试类型，则不建议这样做）。

【讨论】：