How to get the sum of a column

I found this in the guide, but have no idea how to implement the same

yii\db\Query::count(): returns the result of a COUNT query. Other

similar methods include sum($q), average($q), max($q), min($q), which

support the so-called aggregational data query. $q parameter is mandatory

for these methods and can be either the column name or expression.

Say for example I have a table name ‘billing’ with columns:

name amount

charge1 110.00

charge2 510.00

Total - 620.00

How I implement using