  The possible superconducting states of strontium ruthenate (Sr$_2$RuO$_4$)
are organized into irreducible representations of the point group $D_{4h}$,
with a special emphasis on nodes occurring within the superconducting gap. Our
analysis covers the cases with and without spin-orbit coupling and takes into
account the possibility of inter-orbital pairing within a three-band,
tight-binding description of Sr$_2$RuO$_4$. No dynamical treatment if
performed: we are confining ourselves to a group-theoretical analysis. The case
of uniaxial deformations, under which the point group symmetry is reduced to
$D_{2h}$, is also covered. It turns out that nodal lines, in particular
equatorial nodal lines, occur in most representations. We also highlight some
results specific to multiorbital superconductivity. Among other things, we find
that odd inter-orbital pairing allows to combine singlet and triplet
superconductivity whithin the same irreducible representation, that pure
inter-orbital superconductivity leads to nodal surfaces and that the notion of
nodes imposed by symmetry is not clearly defined.
