Question

I have two tables, foo and bar:

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

id is not unique here. Not all IDs in foo have their equivalents in bar and vice versa. I need to count all rows with specific ID in both tables and present them in a new table, e.g.:

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

I’ve tried 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;

But this outputs row with id=1 twice, like

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

I’ve also tried 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;

But this query ignores rows from table bar with ID that is missing in table foo:

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

What am I missing? What would be the right query or the right manual to read?

Thanks.

Answer 1

You may want to try the following:

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;

Test case:

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');

Result:

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