Oracle 23c

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ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';
CREATE TABLE table_name (UserID, Value) AS
SELECT 1, 23 FROM DUAL UNION ALL
SELECT 2, 10 FROM DUAL UNION ALL
SELECT 1, 45 FROM DUAL;
3 rows affected

SELECT MAX(maskedid) OVER (PARTITION BY userid) AS maskedid,
       value
FROM   (
  SELECT CASE ROW_NUMBER() OVER (PARTITION BY userid ORDER BY ROWNUM)
         WHEN 1
         THEN SYS_GUID()
         END AS maskedid,
         userid,
         value
  FROM   table_name
);


MASKEDID
VALUE


0x0BFDD884F75C0A09E06502163E386F05
23

0x0BFDD884F75C0A09E06502163E386F05
45

0x0BFDD884F75D0A09E06502163E386F05
10


SELECT MAX(maskedid) OVER (PARTITION BY userid) AS maskedid,
       value
FROM   (
  SELECT CASE ROW_NUMBER() OVER (PARTITION BY userid ORDER BY ROWNUM)
         WHEN 1
         THEN DBMS_RANDOM.VALUE(0, 1e6)
         END AS maskedid,
         userid,
         value
  FROM   table_name
);


MASKEDID
VALUE


84046.38920350070167103410156941056149
23

84046.38920350070167103410156941056149
45

297835.63547525613908362909300172303338
10

MASKEDID	VALUE
0x0BFDD884F75C0A09E06502163E386F05	23
0x0BFDD884F75C0A09E06502163E386F05	45
0x0BFDD884F75D0A09E06502163E386F05	10

MASKEDID	VALUE
84046.38920350070167103410156941056149	23
84046.38920350070167103410156941056149	45
297835.63547525613908362909300172303338	10