Question

I have two tables:

Table A:

id  name
------------    
1   Scott
2   Dan
3   Sam

Table B:

id  name
------------    
1   Dan
2   Andi
3   Jess

My result needs to be:

Id Name  Found
1  Scott A
2  Dan   C i.e. found in both
3  Sam   A
2  Andi  B
3  Jess  B

I am able to do a UNION to fetch the result but how do I create the Found column?

Answer 1

The way to do this is to use a FULL OUTER JOIN, but since this is not supported in MySQL you can instead use a combination of a LEFT JOIN and INNER JOIN and RIGHT JOIN.

(
    SELECT A.Id, A.Name, 'A' AS Found
    FROM A LEFT JOIN B ON A.Name = B.Name
    WHERE B.Name IS NULL
)
UNION ALL
(
    SELECT B.Id, B.Name, 'B' AS Found
    FROM A RIGHT JOIN B ON A.Name = B.Name
    WHERE A.Name IS NULL
)
UNION ALL
(
    SELECT A.Id, A.Name, 'C' AS Found
    FROM A JOIN B ON A.Name = B.Name
)

In fact, you only need a LEFT and RIGHT JOIN because you can handle the INNER JOIN at the same time as you do one of the other two joins. I think the above demonstrates the principle more clearly, but in practice the following will give better performance:

SELECT A.Id, A.Name, IF(B.Name IS NULL, 'A', 'C') AS Found
FROM A LEFT JOIN B ON A.Name = B.Name
UNION ALL
SELECT B.Id, B.Name, 'B' AS Found
FROM A RIGHT JOIN B ON A.Name = B.Name
WHERE A.Name IS NULL

Result:

Id  Name   Found
1   Scott  A    
2   Dan    C    
3   Sam    A    
2   Andi   B    
3   Jess   B