写分段函数出现问题，求吧友解答

代码如下：
\begin{equation}\label{combination}
\begin{array}{l}
u(x) = \left {\begin{array}{*{20}l}
{u_L (x)} \\
{0} \\
{u_R (x)}\\
{1} \\
\end{array}} \right.\begin{array}{*{20}l}
{$a \leq x \leq b$} \\
{$b \leq x \leq c$} \\
{$c \leq x \leq d$} \\
{otherwise} \\
\end{array} \\
\end{array}
\end{equation}
提示错误
! Missing delimiter (. inserted).
<to be read again>
{
l.317 u(x) = \left {
\begin{array}{*{20}l}
?
如果采用数学环境可以生成
\begin{displaymath}
u(x) = \left\{ \begin{array}{ll}
u_L (x) & \textrm{$a \leq x \leq b$}\\
1 & \textrm{$b \leq x \leq c$}\\
u_R (x) & \textrm{$c \leq x \leq d$}\\
0 & \textrm{otherwise}\\
\end{array} \right.
\end{displaymath}
但是这样没办法给公式自动编号，这是怎么回事