Get sum of column with distinct id in SQL

I am trying to get sum of the amount on distinct ext_id.

Id	Ext_id	amount
1	234	5
2	234	5
3	235	10
4	236	8
5	236	8

Select SUM(amount) from Table1 — this will get me sum of all  => 36

I want just sum of distinct Ext_id which should be 23. Can someone help me with this? THanks in advance!

Answer

You can use two levels of aggregation:

select sum(avg_amount)
from (select ext_id, avg(amount) as avg_amount
      from t
      group by ext_id
     ) x