[SOLVED] How can an AND operator and an OR operator output the same data

How can an AND operator and an OR operator output the same data

How could it be that these two queries, resolve in the same output?

SELECT *
FROM [dbo].[Orders]
where 1=1
AND year(OrderDate) = 1998 
AND MONTH(OrderDate) = 5
OR year(OrderDate) = 1997
AND MONTH(OrderDate) = 4 


SELECT *
FROM [dbo].[Orders]
where 1=1
AND (year(OrderDate) = 1998 AND MONTH(OrderDate) = 5)
OR (year(OrderDate) = 1997 AND MONTH(OrderDate) = 4)

I was expecting them to defer, since the second query clearly genrates 2 populations: orders from 05.1998 and 04.1997. The first query somehow does the same...

Solution

You want to use:

where 1=1
AND 
(
  (
         OrderDate >= '19980501'
     AND OrderDate <  '19980601'
  )
  OR
  (
         OrderDate >= '19970401'
     AND OrderDate <  '19970501'
  )
)

As Larnu suggested, see the Operator Precedence topic to see how AND and OR behave, and how to control the behavior you want by grouping predicates inside parentheses.