多個表的SELECT查詢

Question

我有兩個表，foo和bar：

+----+-----+ +----+-----+ 
| id | val | | id | val | 
+----+-----+ +----+-----+ 
| 1 | qwe | | 1 | asd | 
| 2 | rty | | 3 | fgh | 
+----+-----+ +----+-----+ 

id不是唯一在這裏。 foo中的所有ID在bar中均不相同，反之亦然。我需要計算與特定ID的所有行兩個表中，並在新表中呈現出來，如：

+----+-----------+-----------+ 
| id | count_foo | count_bar | 
+----+-----------+-----------+ 
| 1 |   1 |   1 | 
| 2 |   1 |   0 | 
| 3 |   0 |   1 | 
+----+-----------+-----------+ 

我試過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; 

但這輸出與id=1連續兩次，像

+----+-----------+-----------+ 
| id | count_foo | count_bar | 
+----+-----------+-----------+ 
| 1 |   1 |   0 | <- not good 
| 2 |   1 |   0 | 
| 1 |   0 |   1 | <- not good 
| 3 |   0 |   1 | 
+----+-----------+-----------+ 

我也試過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; 

但這查詢忽略從表bar行用的ID表foo丟失：

+----+-----------+-----------+ 
| id | count_foo | count_bar | 
+----+-----------+-----------+ 
| 1 |   1 |   1 | 
| 2 |   1 |   0 | 
+----+-----------+-----------+ <- can I haz `id=3`? 

我缺少什麼？什麼是正確的查詢或正確的手冊閱讀？

謝謝。

Answer 1

你可能想嘗試以下操作：

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; 

測試用例：

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'); 

結果：

+------+-----------+-----------+ 
| id | count_foo | count_bar | 
+------+-----------+-----------+ 
| 1 |   1 |   1 | 
| 2 |   1 |   0 | 
| 3 |   0 |   1 | 
+------+-----------+-----------+ 
3 rows in set (0.00 sec) 

Answer 2

您是否在尋找OUTER JOIN？