Get ID from a board's category

djondinium 
coreNode.id will return the board id if you are on a board page or message/article page. 

ebonifacio 
Here you can check how you can apply category conttraint in the quries

For Root

select * from boards WHERE root_category.id = 'ABC'

 For Parent

select * from boards WHERE parent_category.id = 'ABC'

For mutiple categories 

select * from boards WHERE root_category.id IN ('ABC', 'XYZ')

Sounds like you need

${coreNode.id}

and get id of the current page.