STDDEV_SAMP(expr)
返回expr表达式的样本标准差
MySQL > select stddev_samp(scan_rows)
from log_statis
group by datetime;
+--------------------------+
| stddev_samp(`scan_rows`) |
+--------------------------+
| 2.372044195280762 |
+--------------------------+
STDDEV_SAMP,STDDEV,SAMP