Oracle 18c

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CREATE TABLE tab_name(id INT, col_name VARCHAR2(100) DEFAULT '0000' NOT NULL)
INSERT INTO tab_name(id) VALUES(1)
1 rows affected

SELECT * FROM tab_name

ID
COL_NAME

1
0000

ALTER TABLE tab_name MODIFY col_name VARCHAR2(100) DEFAULT NULL NULL;
INSERT INTO tab_name(id) VALUES(2)
1 rows affected

SELECT * FROM tab_name

ID
COL_NAME

1
0000

2
null