How to remove duplicate columns from join in SQL - sql

Question

I have the following code
SELECT *
FROM customer
INNER JOIN
(SELECT
customerid, newspapername, enddate, n.publishedby
FROM
newspapersubscription ns, newspaper n
WHERE
publishedby IN (SELECT publishedby
FROM newspaper
WHERE ns.newspapername = n.NewspaperName)
UNION
SELECT
customerid, Magazinename, enddate, m.publishedby
FROM
magazinesubscription ms, magazine m
WHERE
publishedby IN (SELECT publishedby
FROM magazine
WHERE ms.Magazinename = m.MagazineName)) ON customer.customerid = customerid
ORDER BY
customer.customerid;
The customer table has the following:
customerid | customername | customersaddress
This query returns the following result:
customerid | customername | customersaddress | customerid | newspapername | enddate| publishedby
What I actually want is
customerid | customername | customersaddress | newspapername | magazinename | enddate| publishedby
Here, the newspapername field should be blank if the magazinename is present and vice versa. Also, the duplicate field of customerid from the union operations should not be present, while in my result, the value of both the newspapername and the magazinename are put under newspapername title.
How can I do that?

Answer 1

Since you are querying the table with '*', you will always get all the columns in both tables. In order to omit this column, you will have to manually name all columns you DO want to query. To address your other need, you need to simply insert a dummy column to each clause in the union query. Below is an example that should work to allow for what you want -
SELECT customer.customerid, customer.customername, customer.customeraddress, newspapername, magazinename, enddate, publishedby
FROM customer
INNER JOIN
(select customerid, newspapername, null Magazinename, enddate, n.publishedby
from newspapersubscription ns, newspaper n
where publishedby in(select publishedby
from newspaper
where ns.newspapername = n.NewspaperName)
UNION
select customerid, null newspapername, Magazinename, enddate, m.publishedby
from magazinesubscription ms, magazine m
where publishedby in(select publishedby
from magazine
where ms.Magazinename = m.MagazineName))
on customer.customerid = customerid
ORDER BY customer.customerid;

Answer 2

To get the projection you want, build sub-queries of the right shape and UNION them to get the result set. UNION ALL is better than UNION because it avoids a sort: you know you'll get a distinct set because you're joining on two different tables.
select * from (
select customer.*
, n.newspapername
, null as magazinename
, ns.enddate
, n.publishedby
from customer
join newspapersubscription ns
on ns.customerid = customer.customerid
join newspaper n
on n.newspapername = ns.newspapername
union all
select customer.*
, null as newspapername
, m.magazinename
, ms.enddate
, m.publishedby
from customer
join magazinesubscription ms
on ms.customerid = customer.customerid
join magazine m
on m.magazinename = ms.magazinename
)
order by customerid, newspapername nulls last, magazinename ;
Here is the output from my toy data set (which lacks publishedby columns:
CUSTOMERID CUSTOMERNAME NEWSPAPERNAME MAGAZINENAME ENDDATE
---------- -------------------- ---------------------- ---------------------- ---------
10 DAISY-HEAD MAISIE THE DAILY BUGLE 30-SEP-17
30 FOX-IN-SOCKS THE DAILY BUGLE 30-SEP-17
30 FOX-IN-SOCKS THE WHOVILLE TIMES 30-SEP-16
30 FOX-IN-SOCKS GREEN NEWS 31-DEC-17
30 FOX-IN-SOCKS TWEETLE BEETLE MONTHLY 31-DEC-16
40 THE LORAX GREEN NEWS 31-DEC-18
6 rows selected.
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