Calculating sum of parts of column based on another column using QGIS Field Calculator?

Question

I would like to calculate the sum of values in one column, based on values in another column. To illustrate my question I uploaded the picture below. The second column consists of zipcodes, the third total sales. I would like to know what the total sales per zipcode are? So in this case:

What is the sum of column three for column two's value 1023? Output then should be 4 (0+1+1+2+0). The sum for 1024 then should be 11,5 following the same logic.

As this is quite a large dataset I would like to have the QGIS field calculator (or any other tool) calculate a new field which states the total sales in the zipcode. So far I can't seem to get it working and was hoping someone here could help me.

As you can see I have some missing values, is this a problem in calculation? Perhaps good to mention is that it is a table.

Answer 1

Update: QGIS 2.18 (07/07/2017):

In the Field Calculator, you can use the Aggregate function sum which allows you to sum the values of a column based on an expression. You can also, optionally, group and filter features for summation. So in your field calculator, you could simply use:

sum( "Sum", "Sales")

Which does exactly the same as the method using the GroupStats plugin.

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Original post: 02/03/2016

Use the GroupStats plugin which you may need to download from Plugins > Manage and Install Plugins.

Here's an example layer using some of the attributes you have shown:

So let's:

1. Run the GroupStats plugin, select the relevant fields for summing (Sales for rows; Sum for values) and then click Calculate. You should see the results showing the sum of each sale:

   

2. Save the results as a .csv file from the GroupStats toolbar:

   

3. Import the .csv file back into QGIS either by dragging it to the interface or from the toolbar:

   Layer > Add Layer > Add Delimited Text Layer (note that the values are separated by semi-colons)

4. Do a Joins with the shapefile and the .csv file, matching the Sales fields:

   

5. The shapefile should now contain the sum values from the .csv file:

   

You could, if you wish, proceed to use the Save As... option on the shapefile and save it as a new one to keep the joined fields.

Answer 2

If you update to the very latest QGIS version 2.14, you can use virtual layers making a sql statement like:

SELECT "MYTABLE"."MYID", Sum("MYTABLE"."SALES")  AS zip_sale /*:int*/
FROM "MYTABLE"
GROUP BY "MYTABLE"."ZIPCODE"

The UI looks like:

You can also use the DB manager, where you have a query builder behind the sql button:

The MYID column can be used to join the summed sales back to the main table. The join tab is under tabel properties.

Take a look at the QGIS visual changelog under Feature: Virtual layers

Answer 3

A way of doing this is the following:

• In the attribute table, choose select by feature. Type "column-name" = 1023. All fields with the value 1023 are selected.
• Go to vector tools --> Analysing tools --> basic statistics. Choose your layer and the column with the sales. Make sure to check the "selected values only" check-box. The sum will be displayed in the result window. You have to write it down ;)
• repeat the steps for every zip-code