I want a new column called "Summed" containing the summed value per "Populatie" name.
How can I accomplish this?
You could do it several ways, the way I will suggest is to install the plugin "Group Stats" onto your QGIS version.
Open the Group Stats tool under the Vector tab
Then add your attributes (Populatie) that you want to create the Count.
In your case choose your area to sum the counts associated with each entry
Also, there is a tutorial created by Anita Graser AKA @underdark which is available Group Stats tutorial.
You can do this as a field calculation without additional plugins.
Basically, create your new integer/float field to store the sum, and in this case it looks like you want it called "Summed" and then to group the sum by the "Populatie" use the following expression where:
In this case, sum the "Opp" field, but group it by the "Populatie" field.
The syntax from the help says it can take a total of 3 arguments:
sum(expression, group_by, filter)
expression - sub expression of field to aggregate
group_by - optional expression to use to group aggregate calculations
filter - optional expression to use to filter features used to calculate aggregate
The above provides a sum of a value, but you can use the
count() function to count how many times a value appears with something like:
(I would prefer to make a comment to ask questions first but I haven't yet attained the privilege. Apologies for skirting the rules.)
What do you envision the data in the new column looking like? Does each row with equal Populatie end up with the same value? Or are you really picturing a new table with each Populatie (listed once) and the respective sum?
If what you ultimately want is the latter, I would suggest simply exporting the table and performing the calculation in other software, e.g. MS Excel. If you go that route (and need help there), here's how I would do it:
and create a formula (in an adjacent cell) using SUMIF:
=SUMIF([all rows of original Populatie column],[individual cell in unique copies],[all rows of original column you wish to sum])
If you manage your cell references appropriately (absolute references for the first and third arguements, relative for the second), you can copy the formula from one row to the rest.