[GIS] Calculating sum of parts of column based on another column using QGIS Field Calculator

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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.

Example

Best Answer

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.

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:

Layer

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:

    GroupStats plugin

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

    Export to csv

  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:

    Joins

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

    Results

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.

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