JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
By default, the data submissions to forms are stored to JotForm databases and will be accessible to you in the Submissions page. There is no easy way to retrieve these data and use for the validation of the values entered in your form.
By "validate that value exists", do you mean you want to ensure that values entered to a certain field are unique? You can actually set your form such that values entered to a certain field is unique. Please see How to Set Unique Submissions on a Form for more details.
Let us know if you need further assistance.