Aha... they’re in
modx_lexicon_entries. Makes sense because the name (and the description) can be localized.
So if your key name is "mykey", the name is retrievable by this query:
SELECT * FROM modx_lexicon_entries WHERE name='setting_mykey';
And the description by this query:
SELECT * FROM modx_lexicon_entries WHERE name='setting_mykey_desc';
So you can’t really do a join on that table... you have to look up the System Settings "key", then search for a lexicon entry whose name is based off that key. Hindsight = 20/20