Question

我必须将一个带有2个参数的表联接起来，以显示明细的最新更新状态，以检查MAX /(...)/ MIN（...）的完成/进行中。我该怎么办？

我尝试了LEFT JOIN，但它返回了空值

SELECT a.showonly 
     , a.want_this_parameter_lastdate
     , min(a.xdate) start
     , max(a.xdate) stop
     , b.lastdate
  FROM samedatabase a
       LEFT JOIN (SELECT xdate lastdate
                       , want_this_parameter_lastdate 
                    FROM samedatabase 
                   WHERE ROWNUM = 1 
                  ORDER BY xdate DESC) b
              ON a.want_this_parameter_lastdate = b.want_this_parameter_lastdate
 WHERE something IN (SELECT DISTINCT equalsomething 
                       FROM another WHERE input like '...') 
   AND xdate> sysdate - 7
GROUP BY a.showonly, a.want_this_parameter_lastdate, b.lastdate
ORDER BY start ASC;

查询后我得到了

showonly|want_this_parameter|start  |stop   |lastdate
a1      |b1*                |5/7/19 |6/7/19 |(null)
a2      |b2**               |6/7/19 |7/7/19 |(null)
a3      |b1*                |7/7/19 |8/7/19 |(null)
a4      |b2**               |9/7/19 |10/7/19|(null)
a5      |b4                 |10/7/19|11/7/19|(null)

我希望这个（ stop 是a1中使用的最后一个b1， lastdate 是a1中使用的最后一个b1所有表格）

showonly|want_this_parameter|start  |stop   |lastdate
a1      |b1*                |5/7/19 |6/7/19 |12/17/19--Last B1 used
a2      |b2**               |6/7/19 |7/7/19 |11/17/19--Last B2 used
a3      |b1*                |7/7/19 |8/7/19 |12/17/19--Last B1 used
a4      |b2**               |9/7/19 |10/7/19|12/17/19--Last B2 used
a5      |b4                 |10/7/19|11/7/19|12/17/19--Last B4 used

*Same b1
**Same b2

更新我终于明白了，但仍然担心查询时间因为MAX（...）将获取整个表，对不对？我该如何改善此性能。

SELECT a.showonly
     , a.want_this_parameter_lastdate
     , min(a.xdate) start
     , max(a.xdate) stop
     , b.lastdate
  FROM samedatabase a 
       LEFT JOIN (SELECT MAX(xdate) lastdate, want_this_parameter_lastdate 
                    FROM samedatabase 
                  GROUP BY want_this_parameter_lastdate) b
              ON a.want_this_parameter_lastdate = b.want_this_parameter_lastdate
 WHERE something IN (SELECT DISTINCT equalsomething 
                       FROM another 
                      WHERE input like '...') 
   AND xdate> sysdate - 7
GROUP BY a.showonly, a.want_this_parameter_lastdate, b.lastdate
ORDER BY start ASC;

Answer 1

我认为简单的子查询可以使工作更快，从而消除不必要的双重分组：

select showonly, parameter, min(xdate) min_dt, max(xdate) max_dt,
       (select max(xdate) from samedatabase where parameter = s1.parameter) lst_dt
  from samedatabase s1
  where xdate > sysdate - 7
    and something in (select equalsomething from another where input like 'i%')
  group by showonly, parameter

^{dbfiddle demo}

如何将select上的最大/最小日期列与上一个数据所在的最后日期合并

1 个答案: