Lucene doesn’t start with a full view of everything, like a SQL database. Lucene starts with no documents matched, and finds things based on the clauses searched on. This is why:
-Content:xyz
On it’s own doesn’t really work. It knows not to bring in content:xyz, but hasn’t been given any documents to match. The same is true of your query, because it’s placed in a subquery.
-Content:xyz is evaluated first, which gets no docs on it’s own. So then you have, effectively
+Content:dish +(no documents)
It’s useful to think of - as an AND NOT rather than simply a NOT (though don’t take that to imply the +/- and AND/OR/NOT syntax necessarily map to each other directly).
If you want to be able to execute a lonely negative query like that, you need to bring in all documents first. The MatchAllDocsQuery is the best way to accomplish that, something like:
BooleanQuery query = new BooleanQuery();
query.add(new BooleanClause(new MatchAllDocsQuery(), BooleanClause.Occur.SHOULD));
query.add(new BooleanClause(new TermQuery(new Term("Content","xyz")), BooleanClause.Occur.MUST_NOT));
Would be the equivalent of a SQL style query with only a negation for a WHERE clause.
Of course, this isn’t really necessary in the case you’ve listed since:
+Content:dish -Content:xyz
Is perfectly adequate.