How to merge 2 tables into one?

Question

I have in my Google Spreadsheet two separate tabs containing each one a table as showed hereafter:

Tab 1                                 Tab 2
______________________________         _________________________
|Client   |Date         |Sum  |       |Client| Date       |  Sum|
|------------------------------        --------------------------
|Dan      | 27/07/2014  | 5   |       |Sam   | 12/07/2014 | 6   |
|Sam      | 02/07/2014  | 5   |       |Nat   | 12/07/2014 | 6   |
|Alex     | 13/07/2014  | 2   |       |Tom   | 13/07/2014 | 3   |
|Tom      | 13/07/2014  | 10  |       |Dan   | 27/07/2014 | 2   |
-------------------------------        -------------------------|

I would like to get to merge these 2 tables and get rid of doublets by putting all the rows containing the same "Date" and "Client" couple and adding the values of the "Sum" field. Here is what I would like to get:

Client     Date          Sum
Dan        27/07/2014    7
Sam        02/07/2014    5
Sam        12/07/2014    6
Alex       13/07/2014    2
Nat        12/07/2014    6
Tom        13/07/2014    13

As you can see, here I just merged those 2 tables and eliminated the doublets

 1) "Dan"&"27/07/2014" the sum of their "Sum" values making "7"
 2) "Tom"&"13/07/2014, the sum of their "Sum" values making "13"

How can I get it using Query function? Can anyone help me?

Answer 1

You can implement array concatenation using curly braces:

=QUERY({'Tab 1'!A:C;'Tab 2'!A2:C},"select Col1, Col2, sum(Col3) where Col1 != '' group by Col1, Col2 label sum(Col3) 'Sum'",1)