I need a select query

Question

I have a dataset like below in two columns ( col1,col2 in table Mytable)
A | ab
A | cd
A | ef
B | xl
B | ik
E | 23
E | op
C | tk
I need a select query which will return first set of every group which I ask for .
For example, if I ask for data for A , B , the select query will return records
A | ab
B | xl
For example, if I ask for data for B ,E the select query will return records..
B | xl
E | 23
For example, if I ask for data for C the select query will return records..
C | tk
How do I write my select query ?

Answer 1

Something like this ?
SCOTT@orcl> create table test(col1 varchar2(1),col2 varchar2(2));
Table created.
SCOTT@orcl> insert into test values('A','ab');
1 row created.
SCOTT@orcl> insert into test values('A','cd');
1 row created.
SCOTT@orcl> insert into test values('A','ef');
1 row created.
SCOTT@orcl> insert into test values('B','xl');
1 row created.
SCOTT@orcl> insert into test values('B','ik');
1 row created.
SCOTT@orcl> insert into test values('E','23');
1 row created.
SCOTT@orcl> insert into test values('E','op');
1 row created.
SCOTT@orcl> insert into test values('C','tk');
1 row created.
SCOTT@orcl> commit;
Commit complete.
SCOTT@orcl>
SCOTT@orcl> ed
Wrote file afiedt.buf
  1  select * from (
  2  select decode(lag(col1) over (order by col1,rownum),null,col1
  3  ,col1,null
  4  ,col1) col1
  5  ,col2
  6  from test) aaa
  7* where aaa.col1='A'
SCOTT@orcl> /
C CO
A abHTH
Girish Sharma