In the image example below:
Feature Line Layer = Transport - District
Lookup Table = TRAFFIC_IMPACTOR_CODE
What is the most efficient way to "connect" (join/relation/query/field calculator) both tables so the DESCRIPTION column of the Lookup Table 'TRAFFIC_IMPACTOR_CODE' is returned for FROM_TRAFFIC_IMPACTOR_CODE and TO_TRAFFIC_IMPACTOR_CODE of 'Transport - District' feature layer?
A simple QGIS JOIN can only return an answer for ONE column in the 'Transport - District' feature layer.
A Relation is specific only for a project, and is useful only when using the Identify Features Tool as far as I can tell.
A Query (Virtual Layer) has the advantage of displaying a subset of the feature layer, how does one Query based on Both tables?
A Field Calculation (Virtual Column) would be nice to have as a persistent set of answers to multiple column relations between the Feature Layer and the Lookup Table. How does one ask for the DESCRIPTION answer from the Lookup Table for both FROM_TRAFFIC_IMPACTOR_CODE and TO_TRAFFIC_IMPACTOR_CODE columns?