Explanation:
A change to the DEFAULT value affects only subsequent insertions to the table. Existing rows will not be affected.
Incorrect Answers:
1: Column definitions can be altered to add DEFAULT values.
3: Column definitions can be altered to add DEFAULT values. It works for columns with a NUMBER data type also.
4: A change to the DEFAULT value affects only subsequent insertions to the table. Existing rows will not be affected.
OCP Introduction to Oracle 9i: SQL Exam Guide, Jason Couchman, p. 219-224
Chapter 5: Creating Oracle Database Objects

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