Writing should be created at the time of table creation; syntax should follow these examples:

CREATE TABLE Employee
(EmpID NUMBER(4) CONSTRAINT Employee_EmpID_PK PRIMARY KEY
EmpLN VARCHAR2(40) CONSTRAINT Employee_EmpLN_NN NOT NULL)

CREATE TABLE Employee
(EmpID NUMBER(4) EmpLN VARCHAR2(40) CONSTRAINT Employee_EmpLN_NN NOT NULL
CONSTRAINT Employee_EmpID_PK PRIMARY KEY(EmpID))