How to easily remove count=1 on aliased field in SQL? - sql

Question

I have the following data in a table:
GROUP1|FIELD
Z_12TXT|111
Z_2TXT|222
Z_31TBT|333
Z_4TXT|444
Z_52TNT|555
Z_6TNT|666
And I engineer in a field that removes the leading numbers after the '_'
GROUP1|GROUP_ALIAS|FIELD
Z_12TXT|Z_TXT|111
Z_2TXT|Z_TXT|222
Z_31TBT|Z_TBT|333 <- to be removed
Z_4TXT|Z_TXT|444
Z_52TNT|Z_TNT|555
Z_6TNT|Z_TNT|666
How can I easily query the original table for only GROUP's that correspond to GROUP_ALIASES with only one Distinct FIELD in it?
Desired result:
GROUP1|GROUP_ALIAS|FIELD
Z_12TXT|Z_TXT|111
Z_2TXT|Z_TXT|222
Z_4TXT|Z_TXT|444
Z_52TNT|Z_TNT|555
Z_6TNT|Z_TNT|666
This is how I get all the GROUP_ALIAS's I don't want:
SELECT GROUP_ALIAS
FROM
(SELECT
GROUP1,FIELD,
case when instr(GROUP1, '_') = 2
then
substr(GROUP1, 1, 2) ||
ltrim(substr(GROUP1, 3), '0123456789')
else
substr(GROUP1 , 1, 1) ||
ltrim(substr(GROUP1, 2), '0123456789')
end GROUP_ALIAS
FROM MY_TABLE
GROUP BY GROUP_ALIAS
HAVING COUNT(FIELD)=1
Probably I could make the engineered field a second time simply on the original table and check that it isn't in the result from the latter, but want to avoid so much nesting. I don't know how to partition or do anything more sophisticated on my case statement making this engineered field, though.
UPDATE
Thanks for all the great replies below. Something about the SQL used must differ from what I thought because I'm getting info like:
GROUP1|GROUP_ALIAS|FIELD
111,222|,111|111
111,222|,222|222
etc.
Not sure why since the solutions work on my unabstracted data in db-fiddle. If anyone can spot what db it's actually using that would help but I'll also check on my end.

Answer 1

Here is one way, using analytic count. If you are not familiar with the with clause, read up on it - it's a very neat way to make your code readable. The way I declare column names in the with clause works since Oracle 11.2; if your version is older than that, the code needs to be re-written just slightly.
I also computed the "engineered field" in a more compact way. Use whatever you need to.
I used sample_data for the table name; adapt as needed.
with
add_alias (group1, group_alias, field) as (
select group1,
substr(group1, 1, instr(group1, '_')) ||
ltrim(substr(group1, instr(group1, '_') + 1), '0123456789'),
field
from sample_data
)
, add_counts (group1, group_alias, field, ct) as (
select group1, group_alias, field, count(*) over (partition by group_alias)
from add_alias
)
select group1, group_alias, field
from add_counts
where ct > 1
;

Answer 2

With Oracle you can use REGEXP_REPLACE and analytic functions:
select Group1, group_alias, field
from (select group1, REGEXP_REPLACE(group1,'_\d+','_') group_alias, field,
count(*) over (PARTITION BY REGEXP_REPLACE(group1,'_\d+','_')) as count from test) a
where count > 1
db-fiddle