Oracle SQL subtracting between subqueries - sql

Question

I have two queries, the first is this:
SELECT * FROM "HOTELS"
LEFT JOIN "ROOM" ON "HOTELS"."HOTEL_ID"="ROOM"."HOTEL_ID"
LEFT JOIN "ROOM_TYPE" ON "ROOM"."ROOM_T"="ROOM_TYPE"."ROOM_T"
WHERE ("ROOM_TYPE"."ROOM_T"='Two beds' AND "HOTELS"."NAME"='Sunset Hotel');
The second is this:
SELECT * FROM "HOTELS"
LEFT JOIN "RESERVATIONS" ON "HOTELS"."HOTEL_ID"="RESERVATIONS"."HOTEL_ID"
WHERE ("HOTELS"."NAME"='Sunset Hotel' AND "RESERVATIONS"."DATE"='7/20/2016');
(I translated the strings from my native language)
OK, I am running the first one in Oracle's APEX SQL Commands page and it returns 4 entries, which are the rooms I am interested in.
I am running the second and it returns the reserved rooms for the date I am interested in, 1 row result.
Now I want to do the subtraction so that I get left with only the 3 rooms that are not reserved at that date.
But I try different formats that I found after googling and I always get some useless error messages.

Answer 1

You can use a left join and check for o matches:
SELECT *
FROM "HOTELS" h JOIN
"ROOM" r
ON h."HOTEL_ID" = r."HOTEL_ID" JOIN
"ROOM_TYPE" rt
ON r."ROOM_T" = rt."ROOM_T" LEFT JOIN
"RESERVATIONS" r
ON h."HOTEL_ID" r."HOTEL_ID" AND r."DATE" = '7/20/2016'
WHERE h."NAME= 'Sunset Hotel' AND
rt."ROOM_T"='Two beds' AND
r."HOTEL_ID" IS NULL;
You should not need LEFT JOIN for the hotels/rooms/room type connections. The ids should be valid -- and even if they are not, the check on room type requires matches anyway. The LEFT JOIN is needed for RESERVATIONS.

Answer 2

I always get some useless error messages
Error messages are never useless, in fact they are very useful to understand what's wrong your are doing or asking your RDBMS to do.
"RESERVATIONS"."DATE"='7/20/2016'
This is wrong since you are comparing a DATE with a string literal. Always, explicitly convert the string into date using TO_DATE and proper format mask. Or, if you dealing only with the date part and not concerned with the time portion, then use the ANSI date literal which uses a fixed format DATE 'YYYY-MM-DD'
Coming back to your original requirement about the query:
SELECT *
FROM "HOTELS" h
LEFT JOIN "ROOM" r
ON h."HOTEL_ID" = r."HOTEL_ID"
LEFT JOIN "ROOM_TYPE" t
ON r."ROOM_T" = t."ROOM_T"
LEFT JOIN "RESERVATIONS" v
ON ( h."HOTEL_ID" = v."HOTEL_ID"
AND v."DATE" = TO_DATE('2016-07-20', 'YYYY-MM-DD' )
WHERE t."ROOM_T" = 'Two beds'
AND h."NAME" = 'Sunset Hotel'
AND v."HOTEL_ID" IS NULL;

Answer 3

You can use a LEFT JOIN to include the RESERVATIONS table and put the date filter as part of the join condition (the other tables can use INNER JOINs):
SELECT *
FROM "HOTELS" h
INNER JOIN "ROOM" r
ON h."HOTEL_ID" = r."HOTEL_ID"
INNER JOIN "ROOM_TYPE" t
ON r."ROOM_T" = t."ROOM_T"
LEFT JOIN "RESERVATIONS" v
ON ( h."HOTEL_ID" = v."HOTEL_ID" AND v."DATE"= DATE '2016-07-20' )
WHERE t."ROOM_T" = 'Two beds'
AND h."NAME" = 'Sunset Hotel'
AND v."HOTEL_ID" IS NULL;
Then to test for a non-reservation you can check that the RESERVATIONS primary key is NULL (as per the last line).
Also note, that if you are comparing to a DATE data type then if you are using a string value Oracle will do an implicit TO_DATE() using the NLS_DATE_FORMAT parameter as the format mask - if this does not match then it will throw exceptions and if it does work and then the parameter gets changed it will break the query. It would be better to use a date literal value instead (e.g. DATE '2016-07-20').
Finally, you probably don't need to use double quotes around all the table and column names. By default, Oracle will convert column and table names to upper-case (which is what you are using so this isn't an issue) and the only reason to double-quote identifiers is if you want to specify a case-sensitive format (which you should try to avoid) or if you want to use keywords as names (which you also ought to avoid). So, if possible change the RESERVATIONS.DATE column to another name as DATE is a keyword in oracle (but the rest of the column names should not need quotes).

Answer 4

Another option is to use MINUS:
Subquery1
MINUS
Subquery2
https://docs.oracle.com/cd/B19306_01/server.102/b14200/queries004.htm