[SQL] The data are present but when i write the query, the result are zero

Please help me. I just learning sql. When I run my query

select distinct city from customer where region IN ('North', 'East');

the result is zero.

But when I write a query like this

select distinct city from customer where region IN ('North' or 'East');

The result are showing.

But again when i write a query like

select count(*) as num_customer from customer;

the result are showing.

But if I add where

select count(*) as num_customer from customer **where region = 'North' and age between 20 and 30; ** The result are not showing eventhough the data for the query are present.

Please help me. I need to solve this issue for me to complete my assigment. I don't know what to do.

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