How to make a computed field in query within base

Table.Amount = num
calculate new field: Tax = Amount * 0.2

I think it is necessary to use sql mode to set up your query and be able to make a calculated field.

Can someone explain how to go about doing this in SQL? I have a table that has customer purchases entered as they purchase. I want to be able to Query and have a sum of those purchases only for the selected customer.