проблема при создании столбца в существующей таблице

Question

SET SERVEROUTPUT ON SIZE 1000000

DECLARE
    v_col_exists   NUMBER;
BEGIN
    SELECT COUNT (*)
      INTO v_col_exists
      FROM user_tab_cols
     WHERE column_name = 'EFFECTIVE_DATE' AND table_name = 'MEMBERS';

    IF (v_col_exists = 0)
    THEN
        EXECUTE IMMEDIATE 'ALTER TABLE members ADD effective_date DATE';
    ELSE
        DBMS_OUTPUT.PUT_LINE ('The column effective_date already exists');
    END IF;
END;