Constraints fire after DML Execution

CREATE TABLE T_DEPARTMENT (
    DEPARTMENT_ID      NUMBER         PRIMARY KEY NOT NULL,
    DEPARTMENT_NAME    VARCHAR2(50)   NOT NULL);

INSERT INTO T_DEPARTMENT (DEPARTMENT_ID, DEPARTMENT_NAME) VALUES( 1, 'ACCOUNTS'    );
INSERT INTO T_DEPARTMENT (DEPARTMENT_ID, DEPARTMENT_NAME) VALUES( 2, 'CALL CENTER' );
INSERT INTO T_DEPARTMENT (DEPARTMENT_ID, DEPARTMENT_NAME) VALUES( 3, 'PURCHASE'    );
INSERT INTO T_DEPARTMENT (DEPARTMENT_ID, DEPARTMENT_NAME) VALUES( 4, 'MARKETING'   );
INSERT INTO T_DEPARTMENT (DEPARTMENT_ID, DEPARTMENT_NAME) VALUES( 5, 'SALES'       );

SQL> SELECT * FROM T_DEPARTMENT ORDER BY 1;

DEPARTMENT_ID DEPARTMENT_NAME
------------- --------------------------------------------------
            1 ACCOUNTS
            2 CALL CENTER
            3 PURCHASE
            4 MARKETING
            5 SALES
  
UPDATE T_DEPARTMENT SET DEPARTMENT_ID = 4 WHERE DEPARTMENT_NAME = 'PURCHASE';
UPDATE T_DEPARTMENT SET DEPARTMENT_ID = 3 WHERE DEPARTMENT_NAME = 'MARKETING';

The above set of DMLs will not work as it violates the unique key constraint. Try the following DML

UPDATE T_DEPARTMENT SET DEPARTMENT_ID = DECODE(DEPARTMENT_NAME,
                                               'MARKETING', 3,
                                               'PURCHASE',  4 )
 WHERE DEPARTMENT_NAME IN ( 'PURCHASE', 'MARKETING' );

SQL> SELECT * FROM T_DEPARTMENT ORDER BY 1;

DEPARTMENT_ID DEPARTMENT_NAME
------------- --------------------------------------------------
            1 ACCOUNTS
            2 CALL CENTER
            3 MARKETING
            4 PURCHASE
            5 SALES