SQL query multiple joins not working - oracle

Question

I am using oracle database and trying to run the following query but it gives the error:
"ERROR at line 17: ORA-00904: "FRH"."NS": invalid identifier"
What is the problem with it?
Following is the query:
SELECT *
FROM
(SELECT *
FROM ROOMS R
WHERE R.Prix<'50') FRM
JOIN
(SELECT *
FROM
(SELECT *
FROM HOTELS H
WHERE H.CatH=2) FH
JOIN
(SELECT *
FROM RESORTS R
WHERE TypeS='montagne') FR
ON FH.NS=FR.NS) FRH
ON (FRH.NS=FRM.NS AND FRH.NH=FRM.NH);
Thanks in advance

Answer 1

You have way too many nested selects here. Your query can be simplified to:
SELECT *
FROM rooms rm
JOIN hotels ht ON ht.ns = rm.ns AND ht.nh = rm.nh
JOIN resorts rs ON rs.ns = ht.ns
WHERE rm.prix < 50
AND ht.cath = 2
AND ss.types = 'montagne';
I am not entirely sure which tables need to be joined using just the ns column and which need both the ns and nh column because you have obfuscated your query so much and did not show us the table definitions.
Alternatively you can move the restrictions on the joined tables into the join condition. This isn't necessary for the inner joins you are using, but could be needed if you ever want to change that to an outer join:
SELECT *
FROM rooms rm
JOIN hotels ht ON ht.ns = rm.ns AND ht.nh = rm.nh AND ht.cath = 2
JOIN resorts rs ON rs.ns = ht.ns AND rs.types = 'montagne'
WHERE rm.prix < 50;
You should also not compare numbers and strings. Assuming rooms.prix is a number column, the condition R.Prix<'50' is wrong. You need to compare the number to a number r.prix < 50