Suppose the mean and the standard deviation of the data $\left\{x_1, x_2, \cdots, x_9\right\}$ are $\mu$ and $\sigma(\neq 0)$, respectively. After including one more data value $x_{10}$, the mean of the data $\left\{x_1, x_2, \cdots, x_9, x_{10}\right\}$ remains $\mu$. What is the standard deviation of the data $\left\{x_1, x_2, \cdots, x_9, x_{10}\right\}$ ?