Oracle 11g Release 2

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create table mytable (AUTHORS varchar(200))
insert into mytable values('test, test1, test2, test3')
1 rows affected

insert into mytable values('test, test1')
1 rows affected

select left(AUTHORS,charindex(',',AUTHORS,charindex(',',AUTHORS,charindex(',',AUTHORS)+1)+1)-1)
from mytable
ORA-00904: "LEFT": invalid identifier

select NVL(
  SUBSTR(AUTHORS, 1,
    INSTR(AUTHORS,',',1,3) -1),
  AUTHORS)
  from mytable;


NVL(SUBSTR(AUTHORS,1,INSTR(AUTHORS,',',1,3)-1),AUTHORS)


test, test1, test2

test, test1