VARIANCE(expr)
返回expr表达式的方差
MySQL > select variance(scan_rows)
from log_statis
group by datetime;
+-----------------------+
| variance(`scan_rows`) |
+-----------------------+
| 5.6183332881176211 |
+-----------------------+
MySQL > select var_pop(scan_rows)
from log_statis
group by datetime;
+----------------------+
| var_pop(`scan_rows`) |
+----------------------+
| 5.6230744719006163 |
+----------------------+
VARIANCE,VAR_POP,VARIANCE_POP,VAR,POP