Puedes hacerlo como:
--Dataset Preparation
with tab(ID, NAME,Dept_ID) as (Select 1, 'a', '2,3' from dual
UNION ALL
Select 2, 'b','' from dual
UNION ALL
Select 3, 'c' , '1,2' from dual)
--Actual Query
select distinct ID, NAME, regexp_substr(DEPT_ID,'[^,]+', 1, level)
from tab
connect by regexp_substr(DEPT_ID,'[^,]+', 1, level) is not null
order by 1;
Editar:
with tab(ID, NAME,Dept_ID) as (Select 1, 'a', '2,3' from dual
UNION ALL
Select 2, 'b','' from dual
UNION ALL
Select 3, 'c' , '1,2' from dual) ,
--Table Dept
tbl_dept (dep_id,depname) as ( Select 1,'depa' from dual
UNION ALL
Select 2,'depb' from dual
UNION ALL
Select 3,'depc' from dual
) ,
--Seperating col values for join. Start your query from here using with clause since you already have the two tables.
tab_1 as (select distinct ID, NAME, regexp_substr(DEPT_ID,'[^,]+', 1, level) col3
from tab
connect by regexp_substr(DEPT_ID,'[^,]+', 1, level) is not null
order by 1)
--Joining table.
Select t.id,t.name,t.col3,dt.depname
from tab_1 t
left outer join tbl_dept dt
on t.col3 = dt.dep_id
order by 1
