3 个表中的 COUNT 行，包括 MySQL 的零值

Question

我有一个带有 3 个表的 MySQL 5.6 数据库：

job_offer

+----+----------+-------------+-----------+
| id |   name   | position_id | status_id |
+----+----------+-------------+-----------+
|  1 | John     |           1 |         4 |
|  2 | Smith    |           1 |         4 |
|  3 | Williams |           2 |         2 |
+----+----------+-------------+-----------+

位置

+----+----------+
| id |   name   |
+----+----------+
|  1 | frontend |
|  2 | backend  |
+----+----------+

状态

+----+-----------+
| id |   name    |
+----+-----------+
|  1 | contacted |
|  2 | declined  |
|  3 | rejected  |
|  4 | interview |
+----+-----------+

我想构建一个查询，可以按职位和状态计算所有工作机会。 我有这个查询几乎按照我想要的方式执行：

SELECT 
    position.name AS position_name,
    status.name AS status_name
    COUNT(job_offer.id) AS offers
FROM
    job_offer
LEFT OUTER JOIN
    position
        ON job_offer.position_id = position.id
LEFT OUTER JOIN
    status
        ON job_offer.status_id = status.id
GROUP BY
    position_name, status_name

这给了我这个结果：

+---------------+-------------+--------+
| position_name | status_name | offers |
+---------------+-------------+--------+
| frontend      | interview   |      2 |
| backend       | declined    |      1 |
+---------------+-------------+--------+

唯一的问题是我还需要显示与职位相关的所有现有状态，无论是否为 NULL。所以理想情况下它应该是这样的：

+---------------+-------------+--------+
| position_name | status_name | offers |
+---------------+-------------+--------+
| frontend      | contacted   |      0 |
| frontend      | declined    |      0 |
| frontend      | rejected    |      0 |
| frontend      | interview   |      2 |
| backend       | contacted   |      0 |
| backend       | declined    |      1 |
| backend       | rejected    |      0 |
| backend       | interview   |      0 |
+---------------+-------------+--------+

是否有可能通过一个查询来实现？提前感谢您的帮助。

Answer 1

我们可以在position 和status 表之间使用交叉连接方法来生成所有可能的组合。然后，左连接到job_offer 并按位置和状态聚合以查找计数：

SELECT
    p.name AS position_name,
    s.name AS status_name,
    COUNT(jo.id) AS offers
FROM position p
CROSS JOIN status s
LEFT JOIN job_offer jo
    ON jo.position_id = p.id AND
       jo.status_id = s.id
GROUP BY
    p.name,
    s.name
ORDER BY
    p.name,
    s.name;

Demo