SQL JOIN problem joining same table

Here is my mock up data:

Groups:

MAINGROUP SUBGROUP
1         A
2         A
3         A 
1         B
2         B
3         C
1         D
2         D
3         D
2         E
3         E

I'm trying to write a query that will return me one row per SUBGROUP, with one field for each MAINGROUP displaying the group number. If the subgroup does not have a matching group, I want that group field to be NULL.

Here's what I want:

Subgroup  G1  G2  G3
A         1   2   3
B         1   2 
C                 3

Here's my query:

SELECT 
    g.subgroup AS Subgroup, 
    1.maingroup AS G1, 
    2.maingroup AS G2, 开发者_如何学C
    3.maingroup AS G3, 
FROM Groups g
LEFT JOIN(
        SELECT maingroup, subgroup
        FROM Groups
        WHERE maingroup = 1
        ) 1
        ON g.subgroup=1.subgroup
        AND g.maingroup=1.maingroup
LEFT JOIN(
        SELECT maingroup, subgroup
        FROM Groups
        WHERE maingroup = 2
        ) 2
        ON g.subgroup=2.subgroup
        AND g.maingroup=2.maingroup
LEFT JOIN(
        SELECT maingroup, subgroup
        FROM Groups
        WHERE maingroup = 3
        ) 3
        ON g.subgroup=3.subgroup
        AND g.maingroup=3.maingroup
WHERE g.subgroup IN ('A','B','C')

And here's what I'm getting:

Subgroup  G1  G2  G3
A         1     
A             2 
A                 3
B         1     
B             2 
B
C           
C           
C                 3

Any help and advice much appreciated.

Try this:

SELECT g.subgroup AS Subgroup, 
    mg1.maingroup AS G1, 
    mg2.maingroup AS G2, 
    mg3.maingroup AS G3
FROM (SELECT DISTINCT subgroup FROM Groups) g
LEFT JOIN Groups mg1 ON mg1.maingroup = 1 AND mg1.subgroup = g.subgroup
LEFT JOIN Groups mg2 ON mg2.maingroup = 2 AND mg2.subgroup = g.subgroup
LEFT JOIN Groups mg3 ON mg3.maingroup = 3 AND mg3.subgroup = g.subgroup
WHERE g.subgroup IN ('A','B','C');