How can I prevent cartesian product to avoid double counting

Question

I am getting wrong data (cartesian product) when using inner join.

Table t1

PurchaseOrder   CostID  Amount
1                1        4
1                2        3

Table t2

PurchaseOrder   OrderType   ItemId
   1               321        1
   1               321        2
   1               321        3
   1               321        4
   2               128        5
   2               128        6
   3               321        9

Required Output

 PurchaseOrder  Amount
     1            7

My Output

 PurchaseOrder  Amount
     1            28

I am trying to use inner join to get the output but not getting the right data.

Query:

CREATE TEMP TABLE t1
(
PurchaseOrder INT64,
CostID INT64,
Amount INT64
);
INSERT INTO t1
VALUES (1,1,4),(1,2,3);
SELECT *
FROM t1;

CREATE TEMP TABLE t2
(
PurchaseOrder INT64,
OrderType INT64
);
INSERT INTO t2 
VALUES (1,321),(1,321),(1,321),(1,321);
SELECT *
FROM t2;

select t1.PurchaseOrder, sum(amount)
from t1 inner join t2 on t1.PurchaseOrder = t2.PurchaseOrder
where t2.OrderType = 321
group by t1.PurchaseOrder;

Answer 1

I think you just want to check if some PurchaseOrder has an OrderType 321, that is what you use Table 2 for, to do this check. But then to sum the amounts, all information is already in Table 1. So I would write the query as follows:

select PurchaseOrder, sum(amount)
from t1
where exists
 (select * from t2
  where PurchaseOrder=t1.PurchaseOrder
    and OrderType=321)
group by PurchaseOrder