Oracle 11g Release 2

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CREATE TABLE TEST (
    ID_NUM INTEGER,
    NAME INTEGER,
    VALUE INTEGER,
    ITEMS_NUM INTEGER
)
CREATE OR REPLACE FUNCTION RANDOM_UUID RETURN VARCHAR IS
  V_UUID VARCHAR(255);
BEGIN
  SELECT REGEXP_REPLACE(RAWTOHEX(SYS_GUID()), '([A-F0-9]{8})([A-F0-9]{4})([A-F0-9]{4})([A-F0-9]{4})([A-F0-9]{12})', '\1-\2-\3-\4-\5') INTO V_UUID FROM DUAL;
  RETURN V_UUID;
END RANDOM_UUID;
/
ALTER TABLE TEST 
    DROP COLUMN ID_NUM;
ALTER TABLE TEST 
    ADD ID_NUM VARCHAR(255) DEFAULT random_uuid()  NOT NULL;
ORA-04044: procedure, function, package, or type is not allowed here

ALTER TABLE TEST 
    ADD ID_NUM VARCHAR(255)
    DEFAULT REGEXP_REPLACE(RAWTOHEX(SYS_GUID()), '([A-F0-9]{8})([A-F0-9]{4})([A-F0-9]{4})([A-F0-9]{4})([A-F0-9]{12})', '\1-\2-\3-\4-\5')
    NOT NULL;