T-SQL update join 1:N - sql-server

Question

I have two tables:
Table1:
id fee1 fee2
1 0.00 0.00
2 0.00 0.00
Table2:
id fee_no fee
1 A 10.00
1 B 20.00
2 A 80.00
2 B 90.00
SQL:
update a
set a.fee1 = isnull(a.fee1, 0)
+ (case when b.fee_no ='A'
then cast(isnull(b.fee, 0) as decimal(30, 2))
else 0 end),
a.fee2 = isnull(a.fee2, 0)
+ (case when b.fee_no ='B'
then cast(isnull(b.fee, 0) as decimal(30,2))
else 0 end)
from table1 a
inner join table2 b on a.id = b.id
After I executed this SQL, only fee1 of table1 is updated, while fee2 is not. Finally I used two SQL statements to update fee1 and fee2 respectively.
But why doesn't this SQL statement work?
Here is create table statement:
create table table1(
id int null,
fee1 dec(30,2) null,
fee2 dec(30,2) null
)
insert into table1 (id,fee1,fee2)
select 1,0,0 union
select 2,0,0
create table table2(
id int null,
fee_no varchar(10) null,
fee dec(30,2) null
)
insert into table2 (id,fee_no,fee)
select 1,'A',10 union
select 1,'B',20 union
select 2,'A',80 union
select 2,'B',90

Answer 1

The problem with your query is that each row of table1 is updated twice. However when the second update takes place, it operates on the original data of the table. So, fee2 is set back to 0.
To properly UPDATE you need a query like this:
update a
set a.fee1 = isnull(a.fee1, 0)
+ (case when b.fee_no ='A'
then cast(isnull(b.fee, 0) as decimal(30, 2))
else 0 end),
a.fee2 = isnull(a.fee2, 0)
+ (case when c.fee_no ='B'
then cast(isnull(c.fee, 0) as decimal(30,2))
else 0 end)
from table1 a
inner join table2 b on a.id = b.id and b.fee_no = 'A'
inner join table2 c on a.id = c.id and c.fee_no = 'B'