左连接和从表计数

Question

我有 3 table

Table Ploeg 与

PloegID | Name |
5       | name1|
2       | name2|

Table 四舍五入

ID | PloegID | Timestamp
 1 |   5     |  xxxxxx
 2 |   2     |  xxxxxx
 3 |   5     |  xxxxxx
 4 |   5     |  xxxxxx
 5 |   2     |  xxxxxx

Table 平均值为

ID | PloegID | Speed
 1 |   5     |   4
 2 |   2     |   3
 3 |   5     |   6
 4 |   5     |   2
 5 |   2     |   3

我需要的是 table 和

PloegID | Average of Speed for each ploegID | Name | Count of Ploegid in table Round.
   5    |            6                      | name1 |       3
   2    |            3                      | name2 |       2

我有这段代码，但它计算的轮次有误

SELECT g.ploegid, AVG(g.speed) as average,l.name ,count(r.ploegid)   AS rondes 
FROM averages as g
left join Ploeg as l on  l.id = g.ploegid
left join Round as r on   r.ploegid = l.ploegid
GROUP BY ploegid
ORDER BY g.ploegid;

速度正确，但计数不正确。

Answer 1

每个 table 中可能有多行与 ploegid 匹配。因此，您得到的笛卡尔积会导致计算失败。我还建议您以 Ploeg 开始 left joins，因为这似乎是驱动 table:

SELECT l.ploegid, g.average, l.name, r.rondes 
FROM Ploeg l left join
     (select ploegid, avg(g.speed) as average
      from averages g
      group by ploegid
     ) g
     on  l.id = g.ploegid left join
     (select ploegid, count(*) as rondes
      from Round r
      group by ploegid
     ) r
     on r.ploegid = l.ploegid
ORDER BY l.ploegid;

左连接和从表计数

Left join and Count from tables

mysql

count