Question

here is the query

SELECT * FROM customers  
WHERE 
     NOT EXISTS 
     (     
          SELECT 1 FROM brochure_requests     
          WHERE  brochure_requests.first_name = customers.customer_first_name AND    
          brochure_requests.last_name = customers.customer_last_name
     )

This query works just fine but I am not sure why it works. In the NOT EXISTS part SELECT 1 what is the 1 for. When I ran this query

select 1 from test2

Here were the results:

1
-----
1
1
1
1
1
1
1
1
1
1
1
..

How does the not exists query work?

Answer 1

The compiler is smart enough to ignore the actual SELECT in an EXISTS. So, basically, if it WOULD return rows because the filters match, that is all it cares about…the SELECT portion of the EXISTS never executes. It only uses the EXISTS clauses for evaluation purposes

I had this misconception for quite some time since you will see this SELECT 1 a lot. But, I have seen 42, *, etc….It never actually cares about the result, only that there would be one :). The key to keep in mind that SQL is a compiled language, so it will optimize this appropriately.

You could put a 1/0 and it will not throw a divide-by-zero exception…thus further proving that the result set is not evaluated. This is shown in this SQLFiddle

Code from Fiddle:

CREATE TABLE test (i int)
CREATE TABLE test2 (i int)

INSERT INTO test VALUES (1)
INSERT INTO test2 VALUES (1)

SELECT i
FROM test
WHERE EXISTS
(
  SELECT 1/0
  FROM test2
  WHERE test2.i = test.i
)

And finally, more to your point, the NOT simply negates an EXISTS, saying to IGNORE any rows that match