Displaying categories on basis of unique centerId

Question

Hello,
SQL> desc emailCat
Name                                      Null?    Type
CAT_ID                                    NOT NULL NUMBER(14)
ADMIN_ID                                           NUMBER(14)
C_ID                                               NUMBER(14)
CATEGORY                                           VARCHAR2(110)
SQL>In above table c_id is foreign key that presents center Id and each c_id may have many categories(cat_id). I want to display unique c_id with it many cat_id. I think if we make one query of the following statement then it could be achieved. How can I make one query out of following two queries?
select unique(c_id) from emailCat;
select cat_id from emailCat where c_id=? /**on basis of unique c_id of first query**/Please help me
Thanks in anticipation

Answer 1

create table emailCat(
cat_id number(14) primary key,
admin_id number(14) constraint emailCat_fk1 references Admin(admin_id),
c_id number(14) constraint emailCat_fk2 references Centers(c_id),
category varchar2(110)
create sequence emailCat_seq
start with 1 increment by 1;
create or replace trigger emailCat_trig
before insert on emailCat
referencing new as new
for each row
begin
select emailCat_seq.nextval into :new.cat_id from dual;
end;
insert into emailCat(c_id,category) values(786169,'Registered Patients');
insert into emailCat(c_id,category) values(786169,'Newsletter');
insert into emailCat(c_id,category) values(786169,'Hello Testing');Need output like:
select unique(c_id) from emailCat;
select cat_id from emailCat where c_id=?
Thanks for replying and best regards