기존 테이블에 FK 제약조건을 추가하는 ALTER TABLE 구문으로 옳은 것은?

1. 1ALTER TABLE emp ADD CONSTRAINT fk_dept FOREIGN KEY (deptno) REFERENCES dept(deptno)
2. 2ALTER TABLE emp ADD FOREIGN KEY deptno → dept.deptno
3. 3ALTER TABLE emp MODIFY CONSTRAINT ...
4. 4ALTER TABLE emp CREATE CONSTRAINT ...