In QGIS can I show just the lines in a Postgis query? How can I do that?

Question

I have three tables in Postgresql, each table with one layer of lines. I want to show the path of an initial line to another final. Is it possible executing a SQL query? Thanks

My three tables are connected:

Table cables - gid, id, the_geom

Table tubes - gid, id, cable_id, the_geom

Table fibers - gid, id, tube_id, the_geom

If I select the fiber id=1 I want to know (and see in map) the way starting in fiber 1 until the beginning (fiber - tube - cable)

Answer 1

use extention "DB Manager" and there maybe type the SQL-Query (one layer):

    select gid, id, 'cable' as type, the_geom from cables a join tubes b join fibers c on b.id = c.tube_id on a.id = b.cable_id where c.id = 1
    union
    select gid, id, 'tube' as type,`the_geom from tubes d join fibers e on d.id = e.tube_id where e.id = 1
    union
    select gid, id, 'fiber' as type, the_geom from fibers where id = 1;

or type every select (without union) to get three layers.