Add column if not exists

DECLARE v_column_exists NUMBER := 0;
BEGIN
SELECT COUNT(*) INTO v_column_exists FROM all_tab_cols WHERE column_name = 'columnName' AND TABLE_NAME = 'PGBORDEREAU';
IF (v_column_exists = 0) THEN EXECUTE IMMEDIATE 'your sql to add a column';
END IF;
END;
/

e.g.


DECLARE v_column_exists NUMBER := 0;
BEGIN
SELECT COUNT(*) INTO v_column_exists FROM all_tab_cols WHERE column_name = 'EMPCODE ' AND TABLE_NAME = 'EMPLOYEE ';
IF (v_column_exists = 0) THEN EXECUTE IMMEDIATE 'ALTER TABLE EMPLOYEE ADD EMPCODE NUMBER(5) DEFAULT 0';
END IF;
END;
/