SQL query question: SELECT … NOT IN

Question

I am sure making a silly mistake but I can\'t figure what:

In SQL Server 2005 I am trying select all customers except those who have made a reservation before 2 AM.<

Answer 1

SELECT distinct idCustomer FROM reservations
WHERE DATEPART ( hour, insertDate) < 2
  and idCustomer is not null

Make sure your list parameter does not contain null values.

Here's an explanation:

WHERE field1 NOT IN (1, 2, 3, null)

is the same as:

WHERE NOT (field1 = 1 OR field1 = 2 OR field1 = 3 OR field1 = null)

• That last comparision evaluates to null.
• That null is OR'd with the rest of the boolean expression, yielding null. (*)
• null is negated, yielding null.
• null is not true - the where clause only keeps true rows, so all rows are filtered.

(*) Edit: this explanation is pretty good, but I wish to address one thing to stave off future nit-picking. (TRUE OR NULL) would evaluate to TRUE. This is relevant if field1 = 3, for example. That TRUE value would be negated to FALSE and the row would be filtered.