How to remove the blank spaces between two lines in a column - sql

Question

I have a column that returns the following output:
PERSON_NUMBER FEEDBACK
13636 -Very attentive during our sessions
-Very interative session, questions
were asked and answered well.
Nice turn over
Debates were good
I want the output of FEEDBACK Column to look like:
PERSON_NUMBER FEEDBACK
13636 -Very attentive during our sessions
-Very interative session, questions
were asked and answered well.
Nice turn over
Debates were good
I.e. the extra blank spaces between the lines should be removed.

Answer 1

You could do a regex replacement on (?:\r?\n){2,} and replace with just a single CR?LF:
SELECT PERSON_NUMBER,
REGEXP_REPLACE(FEEDBACK,
'(' || chr(13) || '?' || chr(10) || '){2,}',
chr(13) || chr(10)) AS FEEDBACK
FROM yourTable;

Answer 2

If you need to remove all the lines that look empty (e.g. that may also contain spaces, tabs or other non-printable characters), then you may use negation of the [:graph:] class in the regex to find such rows and replace them with empty row and then replace all newline repetitions with single one.
Below is the code:
with a as (
select q'[-Very attentive during our sessions
-Very interative session, questions
were asked and answered well.]' || chr(13) || chr(10) || q'[
]' || chr(10) || q'[
]' || chr(10) || chr(10) || q'[Nice turn over]' as q
from dual
)
select
regexp_replace(
regexp_replace(
q,
/*
Replace LF followed by any non-printable sequence that ends with newline
with single newline
*/
chr(10) || '[^[:graph:]]*(' || chr(13) || '?' || chr(10) || ')',
chr(10) || '\1'
),
/*Then replace newline repetitions*/
'(' || chr(13) || '?' || chr(10) || ')+',
'\1'
) as q
from a
q
-Very attentive during our sessions-Very interative session, questions  were asked and answered well.Nice turn over
db<>fiddle here