Oracle SQL - Select Max Integer Value from UserID String - sql

Question

I want to select the MAX value of the integer associated with the UserID in my Oracle database table to generate the next username for users with similar UserID.
The UserID contains values such as below. There is no fixed pattern of characters before the integer as the string is a username.
TKe10
TKe9
TKe12
TomKelly13
TomKelly9
PJames12
PJames7
I tried using the query below but it always gives TKe9 OR TomKelly9 OR PJames7 as the MAX value.
SELECT * FROM
(SELECT MAX(UserID) from PV_USERS
WHERE REGEXP_LIKE (UserID, '^'|| '<some_user_id>'|| '[^A-
Za-z][0-9]*'));
I have also tried using ORDER BY DESC WHERE ROWNUM<=1 but it also gives the same output.

Answer 1

You need to extract just the numeric part of the ID, which you can do with
regexp_substr(userid, '[0-9]*$')
and then convert that to a number before finding the maximum (otherwise you'll still be doing string comparison, and sorting 9 before 10):
max(to_number(regexp_substr(userid, '[0-9]*$')))
and you probably want to allow for the ID root you're checking to not exist at all yet, which you can do with nvl() or coalesce():
select coalesce(max(to_number(regexp_substr(userid, '[0-9]*$'))), 0) as max_num
from pv_users
where regexp_like(userid, '^'|| 'TomKelly'|| '[0-9]*');
MAX_NUM
----------
13
select coalesce(max(to_number(regexp_substr(userid, '[0-9]*$'))), 0) as max_num
from pv_users
where regexp_like(userid, '^'|| 'PJames'|| '[0-9]*');
MAX_NUM
----------
12
select coalesce(max(to_number(regexp_substr(userid, '[0-9]*$'))), 0) as max_num
from pv_users
where regexp_like(userid, '^'|| 'NewName'|| '[0-9]*');
MAX_NUM
----------
0
... and then add 1 and append back onto the root to get the next ID.
Depending on your business rules, you might want to make the filter case-insensitive.
You should be aware that two sessions performing this operation simultaneously will see the same result, so both would try to create the same ID, e.g. TomKelly14. You either need to serialise this generation operation, or include a fall back - like checking if you get a PK violation when you try to insert the new value into the table and repeating if that happens.

Answer 2

with temp as
(
select 'Tke10' userid from dual
union all
select 'Tke9' userid from dual
union all
select 'Tke12' userid from dual
union all
select 'Tomkelly13' userid from dual
union all
select 'Tomkelly9' userid from dual
union all
select 'Pjames12' userid from dual
union all
select 'Pjames7' userid from dual
)
select A||B from (
select
substr(userid,1,instr(userid,to_number(regexp_substr(userid,'\d+$')))-1) A
,max(to_number(regexp_substr(userid,'\d+$'))) B
from temp
group by substr(userid,1,instr(userid,to_number(regexp_substr(userid,'\d+$')))-1)
)
;