One apparent limitation of using ExSQL() in the separation model is that you can’t use unrelated tables in a field definition. So you’d need to related all your tables to one another.

I just discovered that you can bypass that limitation simply by wrapping your statement in a Let() and declaring a variable that includes wrapping a field in the GetFieldName() function.

So the expression:
Let( field = unrelated::table ; field )
returns an error.

However,
Let( gfn = GetFieldName ( unrelated::table ) ; field = unrelated::table ; field )
does not.

It doesn’t even have to be a field from an unrelated table. Interesting and hopefully helpful.

<a href="http://wingforward.net/interesting-quirk-for-unrelated-field-reference-in-field-definition" rel='nofollow external'>Source</a>