Díky, pokročil jsem sem:

SELECT *, IF
(
users.zastupce < 1 THEN users.name, (SELECT users.prijmeni FROM users WHERE users.zastupce = users.id LIMIT 1)
) datass, sms.id as myID, sms.jmeno as myNAME, users.jmeno as jmeno, users.prijmeni as prijmeni FROM sms LEFT JOIN users us ON us.zalozilID = sms.zalozilID INNER JOIN users ON sms.iduzivatele = users.zastupce OR users.id = sms.iduzivatele AND (odeslano = 1 AND sms.zalozilID = 100 AND platnaSMS = 1 OR users.zastupce = sms.iduzivatele AND odeslano = 1 AND platnaSMS = 1 OR sms.iduzivatele = 100 AND odeslano = 1 AND platnaSMS = 1)
GROUP BY sms.id ORDER BY
sms.id DESC

Ale hlásí to:

Vaše syntaxe je nějaká divná blízko 'THEN users.name, (SELECT users.prijmeni FROM users WHERE users.zastupce = users.' na řádku 3