Question

Continuation

select a.artno, a.name 
from Art a 
inner join store s on a.Artno <> s.Artno` 

Running this query took me more than 1 min producing more 899K rows while was supposed to bring out 7.9K results.

select artno 
from art 
except 
(select artno from store)

This line of code provides me with 7.9K rows which is true for me.

The first codes seems to be working code but takes a hack of a time and produces a large result set. Wondering why?

Answer 1

It’s generally NOT a good idea to use a <> operator with an INNER JOIN unless you really know that you want a lot of records. In other words, the JOIN is a great tool for inclusion, not exclusion.

When you do an INNER JOIN using a <> operator (especially on the keys), the query brings back every combination of art and store records except where the Artno keys match.

So, if your have 4 art records and 5 store records, only one of which had a matching ArtNo value, you would end up with 4 x 5 – 1 = 19 records.

The second query simply displays all art records that aren’t in any store.