Hi there 

Thanks for posting in the Lucidchart Community! Unfortunately it is currently not possible to add notes to individual fields in a table -- Lucidchart only supports adding notes to shapes as a whole. 

However we are always looking for ways to improve your product and your feedback is the best way for us to do that. Would you mind adding your idea to our Product Feedback section? Ideas submitted to this form are passed on to the product development team for their consideration.

For more information on how to add notes to your diagrams take a look at this article and to learn more about how to create ERDs in Lucidchart review the Entity Relationship Diagrams (ERDs) from the Lucidchart Help Center and let us know if you have any questions!