Listing Pairs on same table with different criteria on Oracle Sql - sql

Question

I need to list pairs of customer names as follows;
> CUSTOMER_FIRST_NAME, CUSTOMER_LAST_NAME, CUSTOMER_FIRST_NAME, CUSTOMER_LAST_NAME
such that each pair of customers has the same zip code (which is also in the customers table).
On the task there is a hint which says
HINT: Your output should have four columns: CUSTOMER_FIRST_NAME, CUSTOMER_LAST_NAME, CUSTOMER_FIRST_NAME, CUSTOMER_LAST_NAME
This is what I have written so far:
SELECT DISTINCT CUSTOMER_FIRST_NAME, CUSTOMER_LAST_NAME, CUSTOMER_FIRST_NAME, CUSTOMER_LAST_NAME
FROM CUSTOMERS
WHERE CUSTOMER_ZIP = CUSTOMER_ZIP
But I am not sure how to continue since I've only started learning SQL yesterday. I have also tried to Join the same table which resulted in ambiguity errors.
Update#1:
I've written this code using aliases as suggested by #kpater87
SELECT DISTINCT C1.CUSTOMER_FIRST_NAME, C1.CUSTOMER_LAST_NAME , C2.CUSTOMER_FIRST_NAME, C2.CUSTOMER_LAST_NAME
FROM CUSTOMERS C1
INNER JOIN CUSTOMERS C2
ON C1.CUSTOMER_ZIP = C2.CUSTOMER_ZIP
But even though I have a distinct statement it will show duplicate data. Is this to be expected or am I missing something?

Answer 1

Your updated query looks fine. The only problem in your query is it will be joining also the same records.
If you have a primary key in the table you can improve your query by adding WHERE condition:
SELECT DISTINCT C1.CUSTOMER_FIRST_NAME,
C1.CUSTOMER_LAST_NAME ,
C2.CUSTOMER_FIRST_NAME,
C2.CUSTOMER_LAST_NAME
FROM CUSTOMERS C1
INNER JOIN CUSTOMERS C2
ON C1.CUSTOMER_ZIP = C2.CUSTOMER_ZIP
WHERE C1.PK <> C2.PK;
PK - is a column being a primary key in the table.
If you don't have primary key you can try this one:
SELECT C1.CUSTOMER_FIRST_NAME,
C1.CUSTOMER_LAST_NAME ,
C2.CUSTOMER_FIRST_NAME,
C2.CUSTOMER_LAST_NAME
FROM CUSTOMERS C1
INNER JOIN CUSTOMERS C2
ON C1.CUSTOMER_ZIP = C2.CUSTOMER_ZIP
WHERE C1.CUSTOMER_FIRST_NAME <> C2.CUSTOMER_FIRST_NAME
AND C2.CUSTOMER_LAST_NAME <> C2.CUSTOMER_LAST_NAME
But there still be a problem that in your output you will get e.g.
Mary Smith James Bond
James Bond Mary Smith
To remove permutations:
SELECT C1.CUSTOMER_FIRST_NAME,
C1.CUSTOMER_LAST_NAME ,
C2.CUSTOMER_FIRST_NAME,
C2.CUSTOMER_LAST_NAME,
C1.CUSTOMER_ZIP
FROM T_CUSTOMERS C1
LEFT JOIN T_CUSTOMERS C2
ON (C1.CUSTOMER_ZIP = C2.CUSTOMER_ZIP
AND
C1.CUSTOMER_NUMBER > C2.CUSTOMER_NUMBER );
See also: SQL: self join using each rows only once