Oracle-函数-偏移取值(跨行引用)

    开发中经常用到当前行数据需要和临近行数据进行逻辑处理，这时跨行引用就派上了用场，注意以下结果中PREV_SAL的值：

--LAG分析函数，延后取数，在不使用自联接的情况下同时提供对表的多行数据的访问
SELECT hire_date,last_name,salary,LAG(salary,1,0) OVER (ORDER BY hire_date) AS prev_sal
  FROM employees
  WHERE job_id = ‘PU_CLERK‘
  ORDER BY hire_date;
执行结果：   
HIRE_DATE LAST_NAME                     SALARY   PREV_SAL
--------- ------------------------- ---------- ----------
18-MAY-03 Khoo                            3100          0
24-JUL-05 Tobias                          2800       3100
24-DEC-05 Baida                           2900       2800
15-NOV-06 Himuro                          2600       2900
10-AUG-07 Colmenares                      2500       2600

--LEAD分析函数，向前取数
SELECT hire_date,LEAD(hire_date,1) OVER (ORDER BY hire_date) AS "NextHired" 
  FROM employees
  WHERE department_id = 30
  ORDER BY hire_date;
执行结果:
HIRE_DATE LAST_NAME                 Next Hired
--------- ------------------------- ----------
07-DEC-02 Raphaely                  18-MAY-03
18-MAY-03 Khoo                      24-JUL-05
24-JUL-05 Tobias                    24-DEC-05
24-DEC-05 Baida                     15-NOV-06
15-NOV-06 Himuro                    10-AUG-07
10-AUG-07 Colmenares