mysql - MySQL 中的 Postgres 窗口函数 lag() 等效查询

Question

我有 postgresql 的脚本如下:

select student,
       data, 
       number,
       number - lag(number,1,number) over (partition by student order by id) as output
from the_table
order by student, id

但是我没有用postgresql，我用的是mysql，当我在mysql中尝试那个脚本时，那个脚本是错误的，那么如何在mysql中转换那个脚本呢？

本题与question的关系如下: reduction of each row in the table of database .

Answer 1

您需要使用变量来模拟功能。有关示例，请参见此页面:

http://www.onlamp.com/pub/a/mysql/2007/04/12/emulating-analytic-aka-ranking-functions-with-mysql.html?page=2

-- Oracle
select DEPTNO, AVG(HIRE_INTERVAL)
   2  from  (select DEPTNO,
   3               HIREDATE - LAG(HIREDATE, 1)
   4                             over (partition by  DEPTNO
   5                                   order by HIREDATE)  HIRE_INTERVAL
   6         from EMPLOYEES)
   7   group by DEPTNO

-- MySQL
select DEPTNO, avg(HIRE_INTERVAL)
       -> from (select DEPTNO,
       ->              if (@dept = DEPTNO,
       ->                     datediff(HIREDATE, @hd) +  least(0, @hd := HIREDATE),
       ->                     NULL + least(0, @dept :=  DEPTNO) + (@hd := NULL))
       ->                                                      HIRE_INTERVAL
       ->        from EMPLOYEES,
       ->            (select (@dept := 0)) as a
       ->        order by DEPTNO, HIREDATE) as b
       -> group by DEPTNO;