comments

Delete comments and disable commenting on all nodes of a particular content type

Please TEST these queries before running them on a live site! I do not want to be held responsible if something goes wrong ;)

Here is how a colleague explained his situation (the solution follows):

The trap is that if you create a content type and don’t remember to set comments off, all the nodes you create have to be updated manually to turn it off, even after you change the default in the content type settings. In our case, I imported a ton of records, and only later realized comments were on.