# Labels in subequations

I have the following system of equations

    \begin{subequations}\label{eq}
\begin{align}
&\alpha_x = \dots\label{eqa}\\
&\beta_x = \dots\label{eqb}\\
&\gamma_x = \dots\label{eqc}\\
&\delta_x = \dots \quad\quad \mbox{if $\dots$}\label{eqd1}\\
&\delta_x = \dots \quad\quad \mbox{if $\dots$}\label{eqd2}\\
&\delta_x = \dots \quad\quad \mbox{if $\dots$}\label{eqd3}
\end{align}
\end{subequations}


Now the labels are (1a), (1b), (1c), (1d), (1e), (1f) and calling \ref{eq} I can obtain (1). Being the last three expressions all referred to \delta I want their tags to be in the form (1d-1), (1d-2), (1d-3) and to have the possibility both to call a single one of them (\ref{eqd2} for example) and to call them globally to get (1d). Is there an easy way to do it?

This requirement may require many manual tags,

\documentclass{book}
\usepackage{amsmath}

\begin{document}

\begin{subequations}\label{eq}
\begin{align}
&\alpha_x = \dots\label{eqa}\\
&\beta_x = \dots\label{eqb}\\
&\gamma_x = \dots\label{eqc}
\end{align}
\end{subequations}
\vspace{-\belowdisplayskip}
\vspace{-\abovedisplayskip}
\setcounter{equation}{3}
\renewcommand{\theequation}{\theparentequation\alph{equation}}
\begin{subequations}\label{eqd}
\begin{align}
$\dots$}\label{eqd1}\tag{1d-1}\\
$\dots$}\label{eqd2}\tag{1d-2}\\
$\dots$}\label{eqd3}\tag{1d-3}
\end{align}
\end{subequations}

\setcounter{equation}{1}
\renewcommand{\theequation}{\arabic{equation}}