Oracle 18c

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CREATE TABLE COURSE(
  CRS_CODE VARCHAR(8) NOT NULL,
  CRS_DESCRIPTION VARCHAR(35) NOT NULL,
  CRS_CREDIT INTEGER DEFAULT 4 NOT NULL CHECK(CRS_CREDIT IN (1,2,3,4)),
  PRIMARY KEY (CRS_CODE));
CREATE TABLE CLASS (
  CRS_CODE VARCHAR(8) NOT NULL REFERENCES COURSE(CRS_CODE),
  CLASS_CODE INTEGER NOT NULL CHECK (CLASS_CODE IN (0,1,2,3,4,5,6,7,8)),
  CLASS_TIME VARCHAR(25) NOT NULL,
  CLASS_ROOM CHAR(6),
  PRIMARY KEY (CLASS_CODE)
);

CREATE TABLE STUDENT(
  STU_NUM INTEGER NOT NULL,
  STU_LNAME VARCHAR(25) NOT NULL,
  STU_FNAME VARCHAR(20) NOT NULL,
  STU_INIT VARCHAR(1),
  STU_DOB DATE,
  STU_HRS INTEGER DEFAULT 0 CHECK (STU_HRS>=0 AND STU_HRS<1000),
  STU_CLASS VARCHAR(2) CHECK (STU_CLASS IN ('Fr', 'So', 'Jr', 'Sr', 'Gr')),
  STU_GPA DECIMAL(3,2) DEFAULT 0.00 CHECK (STU_GPA BETWEEN 0.00 AND 4.00),
  STU_PHONE INTEGER,
  PRIMARY KEY (STU_NUM));

CREATE TABLE ENROLL(
  ENROLL_GRADE VARCHAR(1) DEFAULT 'Z' CHECK (ENROLL_GRADE IN('A','B','C','D','F','I','W','Z')),
  STU_NUM INTEGER REFERENCES STUDENT(STU_NUM),
  CLASS_CODE INTEGER REFERENCES CLASS(CLASS_CODE)
);