Bug report #8369

Null propagation of fields in expression based labelling

Added by Patrick Dunford over 9 years ago. Updated over 9 years ago.

Affected QGIS version:master Regression?:No
Operating System: Easy fix?:No
Pull Request or Patch supplied:No Resolution:duplicate
Crashes QGIS or corrupts data:No Copied to github as #:17146


In one of my shapefiles I have an expression based label.
The expression is
"refName" || '\
' || "refDetail" || '\
' || "refSource" || '\
' || "refURL" || '\
' || "refDate"
So it is concatenating five fields in the attribute table along with newlines to format the label.

The problem is that if any of these fields are null, the entire label is not being displayed.
This is presumably from null propagation - one null makes the entire expression result null.

It should be feasible for the expression evaluator to evaluate each field value individually and exclude any nulls from being put into the final result.

Related issues

Duplicates QGIS Application - Feature request #7510: NULL string should be valid in label expressions Closed 2013-04-04
Duplicated by QGIS Application - Bug report #8457: $length fails silently for polygons Closed 2013-08-12


#1 Updated by Nathan Woodrow over 9 years ago

  • OS version deleted (8)
  • Operating System deleted (Windows)

String + NULL = NULL. We don't treat NULL as empty string because it's not correct to do so. NULL and empty mean two different things so are treated differently.

The best way to handle this is using the coalesce function


If refName is null then a empty string is returned. You expression would look like this:

coalesce("refName",'') || '\
' || coalesce("refDetail",'') || '\
' || coalesce("refSource",'') || '\
' || coalesce("refURL",'') || '\
' || coalesce("refDate" ,'')

You can also use the format function in order to clean that up

%5',coalesce("refName",''), coalesce("refDetail",''), coalesce("refSource",''), coalesce("refURL",''), coalesce("refDate" ,''))

#2 Updated by Jürgen Fischer over 9 years ago

  • Resolution set to duplicate
  • Status changed from Open to Closed

duplicate of #7510

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