text: -- create employees4
  CREATE TABLE EMPLOYEES4
  AS SELECT * FROM EMPLOYEES e ;
  
  SELECT * FROM EMPLOYEES4;
  
  SELECT 	* 
  FROM 	EMPLOYEES4 e
  WHERE 	EMPLOYEE_ID = 200
  ;
  
  INSERT INTO EMPLOYEES4 (employee_id, FIRST_NAME, LAST_NAME)
  VALUES (200, 'Ismael', 'Chen');
  
  -- unique index 생성
  CREATE UNIQUE INDEX idx_employee4_id 
  ON EMPLOYEES4(employee_id);
  
  -- non-unique index 생성
  CREATE INDEX idx_employee4_id 
  ON EMPLOYEES4(employee_id);
  
  -- index 삭제
  DROP INDEX idx_employee4_id;
  
  -- 중복 데이터 삭제
  SELECT 	*
  FROM 	EMPLOYEES4 e 
  WHERE 	FIRST_NAME = 'Ismael'
  AND 	LAST_NAME  = 'Chen'
  ;