unexpected output with group by - sql

Question

Here is the DDL and DML:
create table fb_customers (
customer_id int,
customer_name varchar,
product_bought varchar);
insert into fb_customers values
(1, 'james', 'A'),(2, 'james', 'B'), (3, 'james', 'A'), (4, 'james', 'C'), (5, 'ada', 'A'), (6, 'ada', 'A'), (7, 'Tom', 'B'), (8, 'Leo', 'C');
when I select customer_name and product_bought with group by:
select customer_name, product_bought from fb_customers
group by customer_name, product_bought;
the result is surprisingly not grouped by name automatically - I thought with group by the result should be grouped by customer_name and product_bought but we can see james is separated and scattered instead of grouped together.
Now I'd like to find the customers who bought both A and B, and here is my query:
select customer_name from fb_customers
group by customer_name, product_bought
having sum(case when product_bought = 'A' then 1
when product_bought = 'B' then 1
else 0
end) = 2;
and the result is surprising include ada who should not be there:
where am I wrong with these two questions? Thanks

Answer 1

You can try the below - DEMO Here
select customer_name from fb_customers
where product_bought in ('A','B')
group by customer_name
having count(distinct product_bought)=2

Answer 2

please see the below photo
you in your query writed for every customer_name every where product_bought = 'A' then +1 and every where product_bought = 'B' then +1 that in customer_name whit name's ada
ada A +1
ada A +1
since ada have tow 'A' that result sum in query is 2
sum for ada=>1+1=2
and see james
james A +1
james B +1
james A +1
james C 0
since james have tow 'A' and one 'B' that result sum in query is 3
sum for james=>1+1+1=3
query result is ada since just sum ada is 2
and if you want use from your code you can edite as below
select customer_name from fb_customers
group by customer_name
having sum(case when product_bought = 'A' then 1
else 0
end) >= 1
and
sum(case when product_bought = 'B' then 1
else 0
end) >= 1