Comparison operations on xs:boolean in SQL Server 2005

Question

Can anyone explain to me the results of this query:

declare @xml xml;
set @xml = '<node attribute="true">Val</node>';

select
    T.c.query('xs:boolean(@attribute[1])') Value,
    T.c.query('xs:boolean(@attribute[1]) = false') ValueEqualsFalse,
    T.c.query('xs:boolean(@attribute[1]) = true') ValueEqualsTrue,
    T.c.query('xs:boolean(@attribute[1]) != false') ValueNotEqualsFalse,
    T.c.query('xs:boolean(@attribute[1]) != true') ValueNotEqualsTrue
from @Xml.nodes('node') T(c);

The first column, Value, returns true. The rest all return false. So having managed to cast the value to the correct type, how do I actually check it's value?

Answer 1

you need to use fn:false() or false() and fn:true() or true() instead of just writing true or false.

This is a correct example:

T.c.query('xs:boolean(@attribute[1]) = false()') ValueEqualsFalse,
T.c.query('xs:boolean(@attribute[1]) = true()') ValueEqualsTrue,
T.c.query('xs:boolean(@attribute[1]) != false()') ValueNotEqualsFalse,
T.c.query('xs:boolean(@attribute[1]) != true()') ValueNotEqualsTrue

If you don't use the function false(), false is processed as a path expression, i.e. the processor thinks you are querying for a <false /> element. Therefore, there only exists a function for the constants true and false, because this is the only way to distinguish between the boolean constants and a path expression.

More in detail, using the negation would still return false in each example.

This is not what you want (just to demonstrate):

T.c.query('not(xs:boolean(@attribute[1])) = false') ValueEqualsFalse,
T.c.query('not(xs:boolean(@attribute[1])) = true') ValueEqualsTrue,
T.c.query('not(xs:boolean(@attribute[1])) != false') ValueNotEqualsFalse,
T.c.query('not(xs:boolean(@attribute[1])) != true') ValueNotEqualsTrue

the literals false and true both are evaluated to an empty sequence which neither matches the boolean value false() nor the boolean value true().