使用avg和group by进行SQL查询

Question

我在为MySQL编写SQL查询时遇到了一些问题.我有一个具有以下结构的表:

mysql> select id, pass, val from data_r1 limit 10;
+------------+--------------+----------------+
| id         | pass         | val            |
+------------+--------------+----------------+
| DA02959106 | 5.0000000000 |  44.4007000000 |
| 08A5969201 | 1.0000000000 | 182.4100000000 |
| 08A5969201 | 2.0000000000 | 138.7880000000 |
| DA02882103 | 5.0000000000 |  44.7265000000 |
| DA02959106 | 1.0000000000 | 186.1470000000 |
| DA02959106 | 2.0000000000 | 148.2660000000 |
| DA02959106 | 3.0000000000 | 111.9050000000 |
| DA02959106 | 4.0000000000 |  76.1485000000 |
| DA02959106 | 5.0000000000 |  44.4007000000 |
| DA02959106 | 4.0000000000 |  76.6485000000 |

我想创建一个从表中提取以下信息的查询:

id, AVG of 'val' for 'pass' = 1, AVG of 'val' for 'pass' = 2, etc

查询的结果应如下所示:

+------------+---------+---------+---------+---------+---------+---------+---------+
| id         | val_1   | val_2   | val_3   | val_4   | val_5   | val_6   | val_7   |
+------------+---------+---------+---------+---------+---------+---------+---------+
| DA02959106 | 186.147 | 148.266 | 111.905 | 76.3985 | 44.4007 | 0       | 0       |
+------------+---------+---------+---------+---------+---------+---------+---------+

当然,每个唯一的'id'有更多的行.

我已经尝试了一些查询

SELECT id, pass, AVG(val) AS val_1 FROM data_r1 WHERE pass = 1 GROUP BY id;

这将返回正确的结果,但我必须使用"pass"的其他可能值的结果进行扩展(最多7个)

我尝试在AVG中使用嵌套的SELECT,但这不起作用,因为我没有弄清楚如何正确地将它限制为当前的'id'.

然后我创建了Views来表示'pass'= 1,'pass'= 2等每个查询的结果.但是对于大多数id,'pass'的最高值是5.当使用JOIN查询从中获取最终结果时我收到一个空结果集的视图,因为有些视图是空的/没有特定'id'的值.

有任何想法吗？

Answer 1

如果我了解您的需求,请尝试以下方法:

SELECT id, pass, AVG(val) AS val_1 
FROM data_r1 
GROUP BY id, pass;

或者,如果每个id只需要一行,则:

SELECT d1.id,
    (SELECT IFNULL(ROUND(AVG(d2.val), 4) ,0) FROM data_r1 d2 
     WHERE d2.id = d1.id AND pass = 1) as val_1,
    (SELECT IFNULL(ROUND(AVG(d2.val), 4) ,0) FROM data_r1 d2 
     WHERE d2.id = d1.id AND pass = 2) as val_2,
    (SELECT IFNULL(ROUND(AVG(d2.val), 4) ,0) FROM data_r1 d2 
     WHERE d2.id = d1.id AND pass = 3) as val_3,
    (SELECT IFNULL(ROUND(AVG(d2.val), 4) ,0) FROM data_r1 d2 
     WHERE d2.id = d1.id AND pass = 4) as val_4,
    (SELECT IFNULL(ROUND(AVG(d2.val), 4) ,0) FROM data_r1 d2 
     WHERE d2.id = d1.id AND pass = 5) as val_5,
    (SELECT IFNULL(ROUND(AVG(d2.val), 4) ,0) FROM data_r1 d2 
     WHERE d2.id = d1.id AND pass = 6) as val_6,
    (SELECT IFNULL(ROUND(AVG(d2.val), 4) ,0) FROM data_r1 d2 
     WHERE d2.id = d1.id AND pass = 7) as val_7
from data_r1 d1
GROUP BY d1.id

Answer 2

据我了解，您希望每次通过时每个 id 的平均值。解决办法是

SELECT id, pass, avg(value) FROM data_r1
GROUP BY id, pass;