select  substr(xmlcast(xmlagg(xmlelement(last_name,',' || last_name) order by last_name) as clob),2) list
  from  hr.employees
/

LIST
--------------------------------------------------------------------------------
Abel,Ande,Atkinson,Austin,Baer,Baida,Banda,Bates,Bell,Bernstein,Bissot,Bloom,Bul
l,Cabrio,Cambrault,Cambrault,Chen,Chung,Colmenares,Davies,De Haan,Dellinger,Dill
y,Doran,Ernst,Errazuriz,Everett,Faviet,Fay,Feeney,Fleaur,Fox,Fripp,Gates,Gee,Geo
ni,Gietz,Grant,Grant,Greenberg,Greene,Hall,Hartstein,Higgins,Himuro,Hunold,Hutto
n,Johnson,Jones,Kaufling,Khoo,King,King,Kochhar,Kumar,Ladwig,Landry,Lee,Livingst
on,Lorentz,Mallin,Markle,Marlow,Marvins,Matos,Mavris,McCain,McEwen,Mikkilineni,M
ourgos,Nayer,OConnell,Olsen,Olson,Ozer,Partners,Pataballa,Patel,Perkins,Philtank
er,Popp,Rajs,Raphaely,Rogers,Russell,Sarchand,Sciarra,Seo,Sewall,Smith,Smith,Sti
les,Sullivan,Sully,Taylor,Taylor,Tobias,Tucker,Tuvault,Urman,Vargas,Vishney,Voll
man,Walsh,Weiss,Whalen,Zlotkey


SQL>