text: -- EX1
  CREATE TABLE CUSTOMERS (
      CUST_ID VARCHAR(10) PRIMARY KEY,
      CITY VARCHAR(10)
  );
  
  -- EX2
  CREATE TABLE ORDERS (
      CUST_ID VARCHAR(10) ,
      ORDERDT VARCHAR(8),
      NUMORDER NUMBER(5),
      CONSTRAINT FK_ID FOREIGN KEY (CUST_ID) REFERENCES CUSTOMERS(CUST_ID) ON DELETE CASCADE
  );
  
  -- EX3
  INSERT INTO CUSTOMERS VALUES ('AAA', '서울');
  INSERT INTO CUSTOMERS VALUES ('CCC', '서울');
  INSERT INTO CUSTOMERS VALUES ('DDD', '부산');
  
  -- EX4
  INSERT INTO ORDERS VALUES('AAA', '20170120', 1);
  INSERT INTO ORDERS VALUES('BBB', '20170124', 1);
  