Question

I have problem with GROUP BY statement.

I have a table TEST:

And I want to execute following select statement:

select sum(a)
from TEST
group by a+b

Here is the result:

SUM(A)
3
1
3

It works, but I have no idea why. What expression a+b does with GROUP BY statement?

Thanks.

//edit, sorry for that huge image. I will change it

Answer 1

Try this… SELECT 2 + NULL and see what you get.

The answer isn’t 2, it’s NULL.

So, using your example data…

  A  |  B  | A+B
-----+-----+-----
  1  |  0  |  1
  2  | NULL| NULL
  3  |  2  |  5
  1  | NULL| NULL
 NULL|  2  | NULL

The sum of A where A+B is NULL is… 3
The sum of A where A+B is 1 is…… 1
The sum of A where A+B is 5 is…… 3

You can circumvent this using COALESCE(a, 0) + COALESCE(b, 0)