I was checking GitHub for a possible answer, and found a solution.

Apparently you can query the markdown syntax of each block.

So my new query looks like this:
select * from blocks where type = 'i' and subtype = 't' and markdown like '%[ ]%' order by created

And now I filter only the open tasks / open todo's.

Likewise, one can filter the done tasks with markdown like '%[x]%'