I have two sets of data. A large set of lines with a line_ref field, and a small data-only (no geometry) table with a matching line_ref field (let's say for argument's sake that the data-only table marks recently surveyed sections of pipeline). I'm talking a about a one-to-one relationship on this occasion.
I'm using QGIS 2.8
I can specify a table join, and refer to the joined data from the data-only table in styling the lines.
I can do fancy things with input forms based instead on setting up a relation in the project properties.
What I was wondering was whether I could style the lines based on the relation (rather than specifying a join). I'm prepared to be told that this isn't a sensible thing to do if that's the case.... but I'm thinking ahead to when I start to want to style things where there's a one-to-many relationship. Surely at this point table joins start to get more complex?