Working using QGIS as interface using PostGIS connections.
Adding tables (With geometries) in form of shape files from database using Browser panel-¶
Go to QGIS> view> Panels> Browser panel> PostGIS> Expand the connection> give credentials> add the desired file by double clicking on it.
Adding tables (For non-geometry type) using PostGIS connections-¶
Add PostGIS Layers > Give credentials > select "Also list tables with no geometry" > expand public > click on the desired table> add.
Right click/double click on file> go to Join > perform the desired joins - add/subtract the joins.