具有两个关系表的Mysql嵌套查询

Question

Table reports :
id       (int)
catid    (int)
pub_date (datetime)

Table categories :
id           (int)
parent_id    (int)

我想通过 count(id) 和 catid 的 parent_id 分组来获取报告结果。

例如:

 Date           Count(parent_id = 1)    Count(parent_id = 2)  Count(parent_id = 3)
 
2014-02-24            2                            5                      8 

 

我们应该每天只在一行中得到结果。

提前致谢。

Answer 1

如何使用以下查询 ( SQL Fiddle ):

SELECT SUM(CASE WHEN r.id = 1 THEN 1 ELSE 0 END) AS PID_1,
       SUM(CASE WHEN r.id = 2 THEN 1 ELSE 0 END) AS PID_2,
       SUM(CASE WHEN r.id = 3 THEN 1 ELSE 0 END) AS PID_3
FROM reports AS r
INNER JOIN categories AS c ON c.parent_id = r.id
GROUP BY r.pub_date