如何避免兩次連接到同一維表

Question

我有具有task_id, end_time , start_time表task_instances 。

end_time和start_time鏈接到time_table ，該time_table包含Military_hour格式（24小時，范圍為0 -23）。

我想知道所有任務，其中each tasks start time and end_time belong to the same military hour group ( there are 6 groups , each containing 4 hours) 。

  select (floor(td1.military_hour/4)*4) as td1_military_hour_group, 
  (floor(td2.military_hour/4)*4) as td2_military_hour_group, wk_id 
  from task_instances t1
  inner join time_table td1 on t1.end_time = td1.time_id
  inner join time_table td2 on t1.start_time = td2.time_id 
  where  td1_military_hour_group = td2_military_hour_group

是否需要same time_table twice加入same time_table twice 。 我們可以改善查詢，甚至查詢看起來正確嗎？

Answer 1

您不能在WHERE子句中使用派生值td1_military_hour_group和td2_military_hour_group ，否則會出現錯誤，例如“ where子句中的未知列'td1_military_hour_group””。 您需要在WHERE子句中重復計算：

select wk_id,
       (floor(td1.military_hour/4)*4) as td1_military_hour_group, 
       (floor(td2.military_hour/4)*4) as td2_military_hour_group
  from task_instances t1
  inner join time_table td1
    on t1.end_time = td1.time_id
  inner join time_table td2
    on t1.start_time = td2.time_id 
  where (floor(td1.military_hour/4)*4) = (floor(td2.military_hour/4)*4)

否則使用一個普通的表表達式：

WITH cteTime_groups AS (SELECT time_id,
                               floor(military_hour/4)*4 AS military_hour_group
                          FROM time_table)
select wk_id,
       td1.military_hour_group as td1_military_hour_group, 
       td2.military_hour_group as td2_military_hour_group
  from task_instances t1
  inner join cteTime_groups td1
    on t1.end_time = td1.time_id
  inner join cteTime_groups td2
    on t1.start_time = td2.time_id 
  where td1.military_hour_group = td2.military_hour_group

SQLFiddle在這里