你可能想尝试以下操作:
SELECT f.id, f.filename, f.user_id, f.date, f.filesize,
(
SELECT AVG(s.score)
FROM scores s
WHERE s.file_id = f.id
) average_score
FROM files f;
注意,您可以使用AVG()
聚合函数。没有必要将SUM()
除以COUNT()
。
测试用例:
CREATE TABLE files (id int, filename varchar(10));
CREATE TABLE scores (id int, file_id int, score int);
INSERT INTO files VALUES (1, 'f1.txt');
INSERT INTO files VALUES (2, 'f2.txt');
INSERT INTO files VALUES (3, 'f3.txt');
INSERT INTO files VALUES (4, 'f4.txt');
INSERT INTO scores VALUES (1, 1, 10);
INSERT INTO scores VALUES (2, 1, 15);
INSERT INTO scores VALUES (3, 1, 20);
INSERT INTO scores VALUES (4, 2, 5);
INSERT INTO scores VALUES (5, 2, 10);
INSERT INTO scores VALUES (6, 3, 20);
INSERT INTO scores VALUES (7, 3, 15);
INSERT INTO scores VALUES (8, 3, 15);
INSERT INTO scores VALUES (9, 4, 12);
结果:
SELECT f.id, f.filename,
(
SELECT AVG(s.score)
FROM scores s
WHERE s.file_id = f.id
) average_score
FROM files f;
+------+----------+---------------+
| id | filename | average_score |
+------+----------+---------------+
| 1 | f1.txt | 15.0000 |
| 2 | f2.txt | 7.5000 |
| 3 | f3.txt | 16.6667 |
| 4 | f4.txt | 12.0000 |
+------+----------+---------------+
4 rows in set (0.06 sec)
注意@Ignacio's solution产生相同的结果,因此,是另一种选择。
为什么你在两个表中都有'user_id'? – 2010-06-26 05:39:41
在“文件”中,它表示这个文件属于谁,在“分数”中表示谁对这个文件进行了评分。 – Alex 2010-06-26 05:43:00
然后考虑给出字段相关的名称(例如'owner'和'rater')。 – 2010-06-26 05:44:16