[latex] number several equations with only one number

How can I number several equations in a align environment using only one number?

For example

\begin{align}
w^T x_i + b \geqslant 1-\xi_i \text{ if } y_i=1, \nonumber \\
w^T x_i + b \leqslant -1+\xi_i \text{ if } y_i=-1,
\end{align}

The numbering will appear next to the second equation. But it would be better if it appears between the lines of the two equations.

In this case how to label this group of equations for later referring to?

Thanks and regards!

How about something like:

\documentclass{article}

\usepackage{amssymb,amsmath}

\begin{document}

\begin{equation}\label{A_Label}
  \begin{split}
    w^T x_i + b \geqslant 1-\xi_i \text{ if } y_i &= 1, \\
    w^T x_i + b \leqslant -1+\xi_i \text{ if } y_i &= -1
  \end{split}
\end{equation}

\end{document}

which produces: