Problem with "Select by Expression" in QGIS

Question

I am using QGIS 3.10 for Mac and facing some problems with the "Select by Expression" tool. The function I'm using is not selecting any features, even though it should. I have tested it with a few other functions, and the tool works just fine––this particular function seems to be hairy, and I'm not sure why.

I need to select features in the layer that have a certain population density. As such, I am using the following function in "Search by expression":

"TotPop_" / "sq_mi" >= 1000

I'm not sure what's wrong with the function, because it's not any selecting features at all, even though I manually went through and found a couple that apply! (The table is, unfortunately, too long for me to sift through them all manually.)

Does anyone have ideas about what I might be doing wrong?

Answer 1

Because you are asking to select an evaluation of the division expression to get the population density, you need to to add eval expression before apply the density:

I am expecting the following expression to work:

eval("TotPop_" / "sq_mi" >= 1000)

• she_weeds: Not OP, but wondering why division requires eval() and not addition/subtraction/multiplication?
• ahmadhanb: @she_weeds I don't know. But yes it is strange, addition/subtraction/multiplication work with / without eval() except for division.
• she_weeds: Investigating further, I only seem to encounter this issue with dividing fields where one or both fields are integer datatype. If both are decimal/real there it works. It also works if I use to_real() around the division expression. Also, I get erroneous results when using < rather than > with integer fields - it selects all features. I think this might be a bug. I'll lodge an issue...
• ahmadhanb: @she_weeds thanks for reporting. I agree. It looks like a bug in the expression as the issue is only limited to division expression. For the OP question, at least eval() solve the problem.
• she_weeds: Added issue on github here. github.com/qgis/QGIS/issues/35449 Curiously it only seems to be an issue with shapefiles. I did not have the same problem with a number of other data formats.