Generate sequence of alphabets in using oracle SQL query - sql

Question

I have got a task in which I have to generate alphabet sequence on execution of the same oracle query every time.
Example:
When I execute a query first time, it has to generate A
When I execute the same query second time, it has to generate B.
So, it should generate A,B,C,D......Z. Once it reaches Z, it has to generate AA,AB,AC.....AZ
How to compose a query?

Answer 1

Using alphabetical representation for sequence is not a good idea, you should use numerical values. Oracle provides IDENTITY COLUMN for this purpose.
If you actually need an alphabet sequence, then you could create a simple SEQUENCE and a FUNCTION that returns the alphabet sequence of your choice. Using Tom's query in the function:
CREATE SEQUENCE s;
CREATE OR REPLACE FUNCTION get_alpha_seq (seq_val IN NUMBER)
RETURN VARCHAR2 AS
v_val NUMBER;
alpha_seq VARCHAR2(4000);
BEGIN
v_val := seq_val;
SELECT case when i3 > 0 then chr(i3+ascii('A')-1) end ||
case when i2 > 0 then chr(i2+ascii('A')-1) end ||
chr(i1+ascii('A')-1)
INTO alpha_seq
FROM (
SELECT mod((v_val-1),27) i1,
mod( trunc((v_val-0.1)/27), 27) i2,
mod( trunc((v_val-0.1)/27/27), 27 ) i3,
v_val l
FROM dual
)
WHERE i1 <> 0
AND NOT( l>= 27*27 and i2 = 0)
AND NOT( l>= 27*27*27 and i3 = 0);
RETURN alpha_seq;
END;
/
Now call this function with the sequence NEXTVAL as input:
SELECT get_alpha_seq(s.nextval) FROM dual;
Demo:
SQL> SELECT get_alpha_seq(s.nextval) FROM dual;
GET_ALPHA_SEQ(S.NEXTVAL)
------------------------------
A
SQL> SELECT get_alpha_seq(s.nextval) FROM dual;
GET_ALPHA_SEQ(S.NEXTVAL)
------------------------------
B
Similarly, it can return alphabetical sequence like below:
A
B
C
...
Z
AA
AB
...
AY
AZ
BA
BB
...
ZZ
AAA
AAB
...
ZZZ

Answer 2

To make sure only this query uses the sequence we can use the below
create table alpha_seq( t number);
insert into alpha_seq values(0);
then use the below function
CREATE OR REPLACE FUNCTION get_alpha_sequence
RETURN VARCHAR2 AS
pragma autonomous_transaction;
v_value NUMBER;
alpha_seq VARCHAR2(4000);
begin
update alpha_seq set t=t+1;
select t into v_value from alpha_seq;
commit;
with
letters
as
(select chr( ascii('A')+level-1 ) letter
from dual
connect by level <= 26
),
alpha as (select *
from letters
union all
select l1.letter || l2.letter
from letters l1, letters l2
union all
select l1.letter || l2.letter || l3.letter
from letters l1, letters l2, letters l3)
select letter into alpha_seq from (select a.*,rownum rw1 from alpha a
where rownum <=v_value) where rw1=v_value;
return alpha_seq;
END;
Output:-
select get_alpha_sequence from dual;
A
select get_alpha_sequence from dual;
B