QGIS Field Calculator – Creating Conditional Statements

attribute-tableconditionalfield-calculatorif statementqgis

In QGIS 2.8 I have a large data set and I would like to search one field for any values over a certain amount (in this case 20) and if the value is larger than 20, I would like to return a value of 1 in another field, otherwise return a value of 0.

How can I implement it?

Best Answer

The easy way

The most simple way to do this is to create a new field with the expression

"cat" > 20

This expression will evaluate to a boolean True/False which will be represented as an integer 1 or 0.

Virtual Fields

You can also create a virtual field, which will automatically return an updated value in case the values in cat change (e.g. you edit the layer). Remember that the values of virtual fields will not be saved in the dataset and are only visible inside this QGIS project.

More than boolean

If you have more than a simple "greater than", you need to use

CASE 
  WHEN "cat" > 100 THEN 2
  WHEN "cat" > 10 THEN 1
  ELSE 0
END

Related Solutions

[GIS] Writing If-Then Statement in ArcGIS Field Calculator using Python Parser

Using Python, the calculation would look like this:

For the Pre-Logic Script:

def Calc(a,b,c):
 if a == "000":
      return b
 else:
      return c

For the expression:

Calc( !A! , !B! , !C! )

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

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:

So let's:

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:
Save the results as a .csv file from the GroupStats toolbar:
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)
Do a Joins with the shapefile and the .csv file, matching the Sales fields:
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.