Truy vấn tương đương hàm cửa sổ Postgres lag () trong MySQL

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http://www.onlamp.com/pub/a/mysql/2007/04/12/emulation-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;