Seperate Input/Output type for a Table in PostgreSQL - postgresql

Question

I was wondering if it is possible to have the input for a column be one string, with the output being a different string through some dictionary in PostgreSQL. I do know how to use CASE to convert numbers to strings using a SELECT statement, however, I was hoping to create a table such that inputs only require numbers but outputs always give strings.
As an example, for currency USD, CDN and GBP, where 1 = USD, 2 = CDN and 3 = GBP, an example would be:
CREATE TABLE test_table (
currency CHAR (1) CHECK (currency IN ('1','2','3'))
)
Where I could do this:
INSERT INTO test_table (currency)
VALUES ('1')
INSERT INTO test_table (currency)
VALUES ('1')
INSERT INTO test_table (currency)
VALUES ('2')
INSERT INTO test_table (currency)
VALUES ('3')
INSERT INTO test_table (currency)
VALUES ('3')
and the output would look like this:

Answer 1

You can use a CASE expression:
select case currency
when '1' then 'USD'
when '2' then 'CDN'
when '3' then 'GBP'
when '4' then 'EUR'
end as currency
from test_table;
But a better solution would be to create a currency table:
create table currency
(
id integer primary key,
currency_code varchar(3)
);
Then create a foreign key from your base table to the lookup table:
create table test_table
(
...
currency_id integer not null references currency,
...
);
Then use a join to display the code:
select c.code
from test_table t
join currency c on c.id = t.currency_id;