Question

Let’s say I have a table with the following contents: (just some random values)

year
2005
2009
2010
2912
2982
2947

When I query:

SELECT userid FROM tbluser WHERE year!=2005 or year!=2010

It will output all the years. Why is that?

Answer 1

The other answers suggest what to do already, but I’d like you to make the test yourself (funnily, I am doing it now…):

You have

WHERE year!=2005 or year!=2010

Now, check some number for list against that condition. E.g., use 2011:

WHERE 2011!=2005    <-- indeed 2011 is not equal to 2005, so this is TRUE
   or 2011!=2010    <-- indeed 2011 is not equal to 2010, so this is TRUE

insert the preliminary results into your condition:

WHERE TRUE
   or TRUE          <-- if something is TRUE or TRUE, then surely the result
                        is TRUE as well

If you think this further, then the condition is true for every value on the world, because a single value can’t be equal to multiple, different values.

E.g., use 2005:

WHERE 2005!=2005    <-- 2005 _is_ equal to 2005, so this is FALSE
   or 2005!=2010    <-- 2005 is not equal to 2010, so this is TRUE

then

WHERE FALSE
   or TRUE          <-- Read: Where FALSE is true or TRUE is true, summa TRUE.