Oracle 21c

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CREATE TABLE table_a (id, contracts) AS
SELECT 876, 1 FROM DUAL UNION ALL
SELECT 876, 1 FROM DUAL UNION ALL
SELECT 876, 1 FROM DUAL UNION ALL
SELECT 876, 2 FROM DUAL UNION ALL
SELECT 876, 2 FROM DUAL UNION ALL
SELECT 876, 3 FROM DUAL UNION ALL
SELECT 876, 3 FROM DUAL UNION ALL
SELECT 876, 3 FROM DUAL;
8 rows affected

SELECT ID,
       Rtrim(
         Xmlagg(
           Xmlelement(name, CONTRACTS || ',')
           ORDER BY contracts
         ).extract ( '//text()' ).GetClobVal(), ','
       ) AS CONTRACTS
FROM   (SELECT DISTINCT id, contracts FROM TABLE_A)
GROUP BY ID


ID
CONTRACTS


876
1,2,3


SELECT ID,
       LISTAGG(contracts, ',') WITHIN GROUP (ORDER BY contracts) AS CONTRACTS
FROM   ( SELECT DISTINCT id, contracts FROM TABLE_A)
GROUP BY ID


ID
CONTRACTS


876
1,2,3


SELECT ID,
       LISTAGG(DISTINCT contracts, ',') WITHIN GROUP (ORDER BY contracts) AS CONTRACTS
FROM   TABLE_A
GROUP BY ID


ID
CONTRACTS


876
1,2,3