我有两个表,foo
和 bar
:
+----+-----+ +----+-----+
| id | val | | id | val |
+----+-----+ +----+-----+
| 1 | qwe | | 1 | asd |
| 2 | rty | | 3 | fgh |
+----+-----+ +----+-----+
id
在这里不是唯一的。并非 foo
中的所有 ID 在 bar
中都有对应的 ID,反之亦然。我需要计算两个表中具有特定 ID 的所有行并将它们呈现在新表中,例如:
+----+-----------+-----------+
| id | count_foo | count_bar |
+----+-----------+-----------+
| 1 | 1 | 1 |
| 2 | 1 | 0 |
| 3 | 0 | 1 |
+----+-----------+-----------+
我尝试过UNION SELECT
:
SELECT id, COUNT(id) AS count_foo, 0 AS count_bar FROM foo GROUP BY id
UNION SELECT id, 0, COUNT(id) FROM bar GROUP BY id;
但这会输出带有 id=1
的行两次,例如
+----+-----------+-----------+
| id | count_foo | count_bar |
+----+-----------+-----------+
| 1 | 1 | 0 | <- not good
| 2 | 1 | 0 |
| 1 | 0 | 1 | <- not good
| 3 | 0 | 1 |
+----+-----------+-----------+
我也尝试过LEFT JOIN
:
SELECT id, COUNT(foo.id) AS count_foo, COUNT(bar.id) AS count_bar
FROM foo LEFT JOIN bar USING(id) GROUP BY id;
但是此查询会忽略表 bar
中的行,其 ID 在表 foo
中缺失:
+----+-----------+-----------+
| id | count_foo | count_bar |
+----+-----------+-----------+
| 1 | 1 | 1 |
| 2 | 1 | 0 |
+----+-----------+-----------+ <- can I haz `id=3`?
我错过了什么?正确的查询或正确的阅读手册是什么?
谢谢。
最佳答案
您可能想尝试以下操作:
SELECT d.id,
(SELECT COUNT(*) FROM foo WHERE id = d.id) count_foo,
(SELECT COUNT(*) FROM bar WHERE id = d.id) count_bar
FROM ((SELECT id FROM foo) UNION (SELECT id FROM bar)) d;
测试用例:
CREATE TABLE foo (id int, val varchar(5));
CREATE TABLE bar (id int, val varchar(5));
INSERT INTO foo VALUES (1, 'qwe');
INSERT INTO foo VALUES (2, 'rty');
INSERT INTO bar VALUES (1, 'asf');
INSERT INTO bar VALUES (3, 'ghj');
结果:
+------+-----------+-----------+
| id | count_foo | count_bar |
+------+-----------+-----------+
| 1 | 1 | 1 |
| 2 | 1 | 0 |
| 3 | 0 | 1 |
+------+-----------+-----------+
3 rows in set (0.00 sec)
关于sql - 对多个表进行 SELECT 查询,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/3351706/