Oracle lead 和 lga 两个函数的用途和用法

lead   :   返回下一条记录hire_date的值。  
  SELECT last_name, hire_date,
       LEAD (hire_date, 1) OVER (ORDER BY hire_date) AS "NextHired"
  FROM employees
WHERE department_id = 30;
   
  LAST_NAME                                   HIRE_DATE   NextHired 
  -------------------------   ---------   --------- 
  Raphaely                                     07-DEC-94   18-MAY-95 
  Khoo                                             18-MAY-95   24-JUL-97 
  Tobias                                         24-JUL-97   24-DEC-97 
  Baida                                           24-DEC-97   15-NOV-98 
  Himuro                                         15-NOV-98   10-AUG-99 
  Colmenares                                 10-AUG-99  
   
LAG  :   返回前一条记录hire_date的值（没有的话，值为0）。 
SELECT last_name, hire_date, salary,
       LAG (salary, 1, 0) OVER (ORDER BY hire_date) AS prev_sal
  FROM employees
WHERE job_id = 'PU_CLERK';

         
  LAST_NAME                                   HIRE_DATE           SALARY       PREV_SAL 
  -------------------------   ---------   ----------   ---------- 
  Khoo                                             18-MAY-95               3100                     0 
  Tobias                                         24-JUL-97               2800               3100 
  Baida                                           24-DEC-97               2900               2800 
  Himuro                                         15-NOV-98               2600               2900 
  Colmenares                                 10-AUG-99               2500               2600  

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