Sandstone is a sedimentary rock. It is classified this way because of the way it was created. It was created when sand was compressed and eventually became rock.
Igneous rock, when exposed, will weather into smaller particles by mechanical and chemical means. Eventually the particles will be eroded and be transported by wind, water, ice, and gravity and...
Igenous rocks may be broken down by erosion and then the eroded particles could be trasnprorted and deposited in in a sedimentary fill that eventually becomes sedimentary rock.