What is a better way to validate the response coming back from JDBC request?

You seem to try to do several things in one assertion.

Here is how you can try and assert the response::

• Use Message Content assertion to verify the contents of the response. With it you can configure what information you expect in specific response field.
• Use Script assertion to verify response lenght. Depending on your response, there are multiple ways to do this. The simplest one is checking response lenght:
  

  assert messageExchange.response.contentLength < 500

  This is obviously very basic, a script should be custom for what you expect in your response. You can, for example, get a response as XML, count the number of specific nodes in it and fail the assertion if there are too many.