What does this regular expression mean in Oracle?

Question

SELECT  REGEXP_REPLACE(LISTAGG(A.ID, ',') WITHIN GROUP (ORDER BY A.ID), '([^,]+)(,\1)+', '\1')
FROM    TABLE A

I don't know what "\1" means in the above SQL. After creating a list by separating "A.ID" with commas through "LISTAGG", the purpose seems to be to remove the duplicate "A.ID", but I want to know the exact meaning.

For reference, "A.ID" is a NUMBER(4) column type. (eg 1111, 2222...)

Answer 1

It means that you are attempting to replace duplicates.

REGEXP_REPLACE(
  comma_separated_list,
  '([^,]+)(,\1)+',
  '\1'
)

Then:

• ([^,]+) will match one-or-more non-comma characters and store the value in a capturing group.
• ,\1 will match a comma and then the value from that first capturing group.
• (,\1)+ matches a comma and then the value from that first capturing group and matches it all one-or-more times.

However, it does not work reliably. If you have the list 12,123,123,123,123,1234,4444

Then:

• the first match will be:

   12,123,123,123,123,1234,4444 ^^^^^

  at the start and replace it with just 12 giving:

   123,123,123,123,1234,4444 ^^

  It has already gone wrong as the match did not match a complete element and you have lost the first id value.

• the second match will start after the first match will skip the 3, characters and match:

   123,123,123,123,1234,4444 ^^^^^^^^^^^^^^^

  and replace it with 123 giving:

   123,1234,4444 ^^^

  Again, the match was wrong as it did not match a complete element and it is only coincidence that the value output is correct.

• and the final match will be:

   123,1234,4444 ^^^

  replacing it with just 4 and giving the output:

   123,1234444

  Which is very wrong as you are now missing the id 12 and have an incorrect id of 1234444 .

---

What you should probably be doing is filtering the duplicates before aggregating.

In newer Oracle versions it is simply:

SELECT  LISTAGG(DISTINCT ID, ',') WITHIN GROUP (ORDER BY ID)
FROM    TABLE

or in older versions:

SELECT  LISTAGG(ID, ',') WITHIN GROUP (ORDER BY ID)
FROM    (
  SELECT DISTINCT id FROM TABLE
)

If you did want to use regular expressions (which will be a more inefficient than using DISTINCT ) then you can double up the delimiters and ensure you always match complete elements using (,[^,]+,)\1+ but then you also need to remove the repeated delimiters after de-duplication (which makes an inefficient solution even more inefficient):

SELECT  TRIM(
          BOTH ',' FROM
          REPLACE(
            REGEXP_REPLACE(
              LISTAGG(',' || ID || ',') WITHIN GROUP (ORDER BY ID),
              '(,[^,]+,)\1+',
              '\1'
            ),
            ',,',
            ','
          )
        ) AS no_duplicates
FROM    TABLE_NAME

db<>fiddle here