JOIN and the ON clause (NULL values)

Question

I have two tables

tableA (id,randomNumber) (2,1),(3,5),(3,2),(6,0),(8,0),...
tableB (id,randomNumber2) (0,2),(1,3),(2,0),(4,2)...

Now I'd like to fetch id, randomnumber and randomnumber2 where ID IN (0,1,2,3) from both tables.

So that should return:

(2,1), (3,5), (3,2), (0,2), (1,3), (2,0) 

I tried constructions with JOIN and LEFT JOIN.

The problem is when the ON clause (JOIN tableB b on a.id=b.id) is not true, it returns NULL for the id. But I also need to know the NULL value. For example for tableB it will return (NULL,2) for the first result.

(2,1), (3,5), (3,2), (NULL,2), (NULL,3), (2,0) 

How can I also get the NULL value, should I be using something else ?

Answer 1

JOIN s are used when the tables have related data. Here, the 2 tables have nothing to do with each other. A better choice would be to use a UNION .

SELECT id, randomNumber
FROM tableA
WHERE ID IN (0,1,2,3)

UNION ALL

SELECT id, randomNumber2
FROM tableB
WHERE ID IN (0,1,2,3)

Or, just use 2 separate queries and combine the results.

Answer 2

You can use a UNION with a subquery:

SELECT id, rn
FROM (
    SELECT Id, randomNumber rn
    FROM tableA
    UNION
    SELECT Id, randomNumber2
    FROM tableB
) t
WHERE id IN (0,1,2,3)

You may want to use a UNION ALL if you want duplicate values.