Combine 2 tables in SQL including uncommon fields - sql

Question

I have two tables as follows:
table 1 table 2
id name count1 | id name count2
1 x 2 1 x 1
2 y 3 4 y 1
3 z 1 5 z 2
Result expected:
id name count1 count2
1 x 2 1
2 y 3 0
3 z 1 0
4 y 0 1
5 z 0 2
SQL query tried:
SELECT table1.id as id, table1.name as name,
table1.count1 as count1, table2.count2 as count2
FROM table1
LEFT JOIN table2 on table1.id = table2.id and table1.name = table2.name
I feel like this is wrong because I am not getting the expected result. I am unsure of which join to use since I am new. Any help here would be much appreciated.
Thank you very much

Answer 1

You can use UNION ALL within the subquery and then aggregate in the main query as
SELECT id, name, SUM(count1) AS count1, SUM(count2) AS count2
FROM
(
SELECT id, name, count1, 0 AS count2
FROM table1
UNION ALL
SELECT id, name, 0, count2
FROM table2
) q
GROUP BY id, name

Answer 2

You can use a full join:
SELECT COALESCE(t1.id, t2.id) as id, COALESCE(t1.name, t2.name) as name,
COALESCE(t1.count1, 0) as count1, COALESCE(t2.count2, 0) as count2
table1.count1 as count1, table2.count2 as count2
FROM table1 t1 FULL JOIN
table2 t2
ON t1.id = t2.id and t1.name = t2.name;
If your database does not support full join, then you basically have two options. One only uses LEFT JOIN:
SELECT ni.id, ni.name,
COALESCE(t1.count1, 0) as count1, COALESCE(t2.count2, 0) as count2
table1.count1 as count1, table2.count2 as count2
FROM ((SELECT id, name FROM table1
) UNION -- on purpose to remove duplicates
(SELECT id, name FROM table2
)
) ni LEFT JOIN
table1 t1
ON t1.id = ni.id AND ni.name = t1.name
table2 t2
ON ni.id = t2.id and ni.name = t2.name;
The second method is uses UNION ALL, NOT UNION:
SELECT t1.id, t1.name, t1.count1, COALESCE(t2.count2, 0) as count2
FROM table1 t1 LEFT JOIN
table2 t2
ON t1.id = t2.id and t1.name = t2.name
UNION ALL
SELECT t2.id, t2.name, 0, t2.count2
FROM table2 t2 LEFT JOIN
table1 t1
ON t1.id = t2.id and t1.name = t2.name
WHERE t1.id IS NULL;
I have no idea where the "canonical" approach using UNION comes from. I think it is a thought exercise on being close. However, it is a very poor solution for the following reasons:
UNION adds additional overhead for removing duplicates. And that is not part of FULL JOIN functionality.
The lack of WHERE clause on the second query ensures that there are duplicates if there are any matches.
The use of UNION removes duplicates within each subquery. Whether desirable or not, that is not how FULL JOIN works.