查找与另一列中的特定行值相关的列中的相同行数

Question

我正在尝试进行查询，以了解有多少人在状态3中有活动。我认为我需要COUNT或SUM，但是自从我之前做了很长时间后我已经尝试过搜索很多，但我仍然感到困惑。假设我有以下内容。

活动表：

activity_id  status
1             3
2             3
3             1

Person_activity表：

person_id  activity_id
1            1
1            2
2            2
2            3 
3            3

我想要输出查询：

person_id   no_of_status_3
1             2
2             1
3             0

任何建议都将不胜感激。提前谢谢。

Answer 1

这应该做：

SELECT  PA.person_id,
        COUNT(A.activity_id) no_of_status_3
FROM Person_activity PA
LEFT JOIN ( SELECT *
            FROM Activity
            WHERE status = 3) A
    ON PA.activity_id = A.activity_id
GROUP BY PA.person_id

Here is an sqlfiddle以及此演示。结果是：

╔═══════════╦════════════════╗
║ PERSON_ID ║ NO_OF_STATUS_3 ║
╠═══════════╬════════════════╣
║         1 ║              2 ║
║         2 ║              1 ║
║         3 ║              0 ║
╚═══════════╩════════════════╝

Answer 2

SELECT pa.person_id, count(a.activity_id)
FROM person_activity pa
LEFT JOIN activity a ON (pa.activity_id = a.activity_id AND a.status = 3)
GROUP BY pa.person_id