How to validate an entered value against a sql table on submit click event in k2 SmartForms?

I'm not exactly understand what exactly do you mean with validate.... 

But from the comparison point of view... basically I can see a loop trough a Table. the table has his own SMO. 

Basically once a text field is entered and on click a rule will loop trough this SMO and check for each of the records.

Depending of what you want [ Validate ] Equal, Not Equal, Greaters Than, LessThan... Older Than... etc....

You will return a result, perhaps another action... that could be rendered with Unbounded Rules.

Check this post, I think might help you to get the idea:

http://community.k2.com/t5/K2-blackpearl/Is-it-possible-to-compare-two-SMOs-Loop-trough-all-items-and/m-p/87694#M27254

Hope this helps Dino.