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Databases Querying Relational Databases Set Operations Review and Practice

Wen Yan Cheong
Wen Yan Cheong
1,482 Points
Posted by Wen Yan Cheong
Wen Yan Cheong
1,482 Points

my query is not giving the correct results

for this question

-- Create a list of unique books. -- Books that are in the north or south location, but not in both locations.

i did this:

SELECT books_north.title FROM books_north EXCEPT SELECT books_south.title FROM books_south UNION ALL SELECT books_south.title FROM books_south EXCEPT SELECT books_north.title FROM books_north

but i am only getting 'Congo Jurassic Park Sphere"

I should be getting " Armada Ready Player One The Martian Congo Jurassic Park Sphere""

i do get 'Armada Ready Player One The Martian' correctly with 'SELECT books_north.title FROM books_north EXCEPT SELECT books_south.title FROM books_south' but somehow when i use UNION ALL i am only getting results for the 2nd query instead of joining them

1 Answer

Steven Parker
Steven Parker
230,970 Points
Steven Parker
Steven Parker
230,970 Points

These operations are occurring in sequence, but your intention is perform both EXECPTs before the UNION.

One way you can enforce that order is by using a subquery:

SELECT books_north.title FROM books_north EXCEPT SELECT books_south.title FROM books_south
UNION ALL
SELECT * FROM
( SELECT books_south.title FROM books_south EXCEPT SELECT books_north.title FROM books_north )