Oracle 18c

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
create table Employees
(
EMPLOYEE_ID NUMBER GENERATED BY DEFAULT AS IDENTITY NOT NULL, 
NAME VARCHAR2(30) NOT NULL, 
PHONE_NUMBER VARCHAR(12) CHECK(REGEXP_LIKE(PHONE_NUMBER, '\d{3}-\d{3}-\d{4}')) NOT NULL, 
SALARY NUMBER NOT NULL, 
SENIORITY NUMBER NOT NULL, 
PRIMARY KEY(EMPLOYEE_ID)
);
insert into Employees (NAME, PHONE_NUMBER, SALARY, SENIORITY) VALUES
('Bill', '430-123-5555', 10000, 1)
1 rows affected

SELECT * 
FROM Employees


EMPLOYEE_ID
NAME
PHONE_NUMBER
SALARY
SENIORITY


1
Bill
430-123-5555
10000
1


insert into Employees (EMPLOYEE_ID, NAME, PHONE_NUMBER, SALARY, SENIORITY) VALUES
(NULL, 'Jim', '555-234-1234', 20000, 2)
ORA-01400: cannot insert NULL into ("FIDDLE_DHXZUMCTRWNIJMOWWEVJ"."EMPLOYEES"."EMPLOYEE_ID")

SELECT * 
FROM Employees


EMPLOYEE_ID
NAME
PHONE_NUMBER
SALARY
SENIORITY


1
Bill
430-123-5555
10000
1


ALTER TABLE Employees MODIFY EMPLOYEE_ID NUMBER GENERATED BY DEFAULT ON NULL AS IDENTITY
insert into Employees (EMPLOYEE_ID, NAME, PHONE_NUMBER, SALARY, SENIORITY) VALUES
(NULL, 'Jim', '555-234-1234', 20000, 2)
1 rows affected

SELECT * 
FROM Employees


EMPLOYEE_ID
NAME
PHONE_NUMBER
SALARY
SENIORITY


1
Bill
430-123-5555
10000
1

2
Jim
555-234-1234
20000
2