how to use union query on the same table and different group by in mysql?

Question

I have sql query like this:

select a.x, a.y, sum(a.foo) as foo_sum
from a
group by a.x

and after running this query, I can access to foo_sum.

I want to do something like this:

select a.x, a.y, sum(a.foo) as foo_sum
from a
group by a.x
union
select a.x, a.y, sum(a.bar) as bar_sum
from a
group by a.y

but after running this one, my program dosent know foo_sum, anybody knows why?

thank you.

Edit : I want to have 4 column x, y, foo_sum and bar_sum, is this possible?

Answer 1

I don't think this is possible, because your number of unique x values might not be the same as the number of unique y values.

For example, suppose a was like this:

+------+------+------+------+
| x    | y    | foo  | bar  |
+------+------+------+------+
|    1 |    1 |    3 |    4 |
|    1 |    2 |    3 |    6 |
|    1 |    1 |    3 |    6 |
|    1 |    2 |    8 |    3 |
|    2 |    1 |    7 |   32 |
|    2 |    2 |    7 |   34 |
|    2 |    3 |    8 |    3 |
+------+------+------+------+

The first query ( foo_sum GROUP BY x ) would give:

+------+------+---------+
| x    | y    | foo_sum |
+------+------+---------+
|    1 |    1 |      17 |
|    2 |    1 |      22 |
+------+------+---------+

The second ( bar_sum GROUP BY y ) gives:

+------+------+---------+
| x    | y    | bar_sum |
+------+------+---------+
|    1 |    1 |      42 |
|    1 |    2 |      43 |
|    2 |    3 |       3 |
+------+------+---------+

How can you mash the bar_sum column on the end of the foo_sum table? There is no corresponding foo_sum value for (x,y)=(2,3), but there is a corresponding bar_sum .

The best you could achieve would be something like this:

+------+------+---------+-------+
| x    | y    | bar_sum |foo_sum|
+------+------+---------+-------+
|    1 |    1 |      42 |    17 |
|    1 |    2 |      43 |  NULL |
|    2 |    3 |       3 |  NULL |
|    2 |    1 |    NULL |    22 |
+------+------+---------+-------+

To achieve this the only way I can think of is using a FULL OUTER JOIN . Note that doing SELECT x, y, SUM(foo), SUM(bar) FROM a GROUP BY x,y won't give the same results as it groups differently.

SELECT t1.x,t1.y,foo_sum,bar_sum
FROM
(SELECT x, y, SUM(foo) as foo_sum
FROM A
GROUP BY x) t1
FULL OUTER JOIN
(SELECT x, y, SUM(bar) as bar_sum
FROM A
GROUP BY y) t2
ON t1.x=t2.x AND t1.y=t2.y

This makes sure all the x,y combinations from each table is inserted, even if there are no corresponding x,y in the other table.

However MySQL has no FULL OUTER JOIN , and you have to simulate a A FULL OUTER JOIN B by UNION ing a LEFT and RIGHT join together:

SELECT ...
 FROM A LEFT JOIN B ON ...
UNION
SELECT ...
 FROM A RIGHT JOIN B ON ...

In your case this translates to the very ugly:

SELECT t1.x,t1.y,foo_sum,bar_sum
FROM
(SELECT x, y, SUM(foo) as foo_sum
FROM A
GROUP BY x) t1
LEFT JOIN                        -- need FULL OUTER JOIN
(SELECT x, y, SUM(bar) as bar_sum
FROM A
GROUP BY y) t2
ON t1.x=t2.x AND t1.y=t2.y

UNION

SELECT t2.x,t2.y,foo_sum,bar_sum
FROM
(SELECT x, y, SUM(foo) as foo_sum
FROM A
GROUP BY x) t1
RIGHT JOIN                        -- need FULL OUTER JOIN
(SELECT x, y, SUM(bar) as bar_sum
FROM A
GROUP BY y) t2
ON t1.x=t2.x AND t1.y=t2.y

This is highly inefficient ! I suggest you instead do the grouping on the PHP side (or whatever other language you are using with MySQL) than the SQL side.

Who knows, there may be a more efficient way to do this since all you want is SUM s - there could be a clever way to group and sum to achieve your effect.

Answer 2

Is this same as:

select a.x, a.y, sum(a.foo) as foo_sum, sum(a.bar) as bar_sum 
from a 
group by a.x ,a.y