Question

I have a scenario where there are two tables (tables A and B) linked in a one to many relationship. For a row in table A, the maximum number of linked rows in B is two, and these two rows (if they exist) are different from each other through a type column whose value is either x or y.

Aid | Name                            Bid  | type | Aid
1   | name1                             1  |  x   | 1
2   | name2                             2  |  x   | 2
3   | name3                             3  |  y   | 2

Now, what I want is to have a join query for the two tables in such a way that all rows in A will be displayed (no repetition) and two columns called type x and type y will hold a boolean / integer value to show the existence of types x and y for each row in A. i.e,

Aid | Name  | Type X | Type Y |
1   | name1 |   X    |  NULL  |
2   | name2 |   X    |   Y    |
3   | name3 |  NULL  |  NULL  |

My DBMS is MySql.

Thanks.

Answer 1

You have to use two joins:

SELECT A.*, b1.type AS typeX, b2.type as typeY
FROM A
LEFT JOIN B b1
ON A.aid = b1.aid
AND b1.type = 'x'
LEFT JOIN B b2
ON a.aid = b2.aid
AND b2.type = 'y'