Getting unique values in a QGIS Expression

Question

I'm using QGIS 3.4 attribute form to edit the widget type. I'm trying to retrieve "cab_no." field values (only unique values) from 'U_Cabinet' layer to use it in "fdt#" field in the 'Distribution2' layer. as shown in the image below

I've been trying to retrieve unique values by using the expression builder.

I tried the expression array_distinct(array("cab_no."))

where "cab_no." is the field who has the values.

but there is no result. as shown in the image below.

Am I missing something here??

Answer 1

You must use array_agg:

 array_distinct(array_agg( "cab_no" ))

This will return an array with distinct values found in your layer with the field cab_no.

Answer 2

The filter expression requests a true/false output. The function array_distinct() gives you an array with distinct values as an output, plus you need to use array_agg first, as @etrimaille pointed out. To get a true/false you can try something like:

"id" = array_first( array_agg( "id", group_by:= "cab_no."))

For id you have to insert your unique id column of the U_Cabinet layer. If your u_cabinet layer is big, this can get quite slow.

I think it helps to look at your problem from a different angle. For your purpose I would recommend to use the Widget Type "Value Map" -> Load Data from Layer -> Value and Description are both cab_no.