Question

考虑以下两个（假设的）表

温度

* day
* time
* lake_name
* station
* temperature_f

Temperature_summary

* day
* lake_name
* station
* count_readings_over_75f
* count_readings_below_75f

如何编写 SQLite 触发器来更新insert上的temperature_summary表。我想增加点数。

谢谢你，杰夫

Answer 1

这假设您在当天插入温度之前已经创建了day / lake_name / station的记录。当然，您可以添加另一个触发器来执行此操作。

create trigger Temperature_count_insert_trigger_hi after insert on Temperature
  when new.temperature_f >= 75
  begin
    update Temperature_summary set count_readings_over_75f = count_readings_over_75f + 1
    where new.day = day and new.lake_name = lake_name and new.station = station;
  end;

create trigger Temperature_count_insert_trigger_lo after insert on Temperature
  when new.temperature_f < 75
  begin
    update Temperature_summary set count_readings_below_75f = count_readings_below_75f + 1
    where new.day = day and new.lake_name = lake_name and new.station = station;
  end;

您可以将这些组合成一个稍微复杂的触发器

create trigger Temperature_count_insert_trigger after insert on Temperature
  begin
    update Temperature_summary
    set count_readings_below_75f = count_readings_below_75f + (new.temperature_f < 75),
      count_readings_over_75f = count_readings_over_75f + (new.temperature_f >= 75)
    where new.day = day and new.lake_name = lake_name and new.station = station;
  end;

要确保Temperature_summary中有一行要更新（a）在Temperature_summary的(day, lake_name, station)上创建一个唯一索引，或者将这些列作为主键，并且（b）在触发器中执行插入或忽略这样：

create trigger Temperature_count_insert_trigger after insert on Temperature
  begin
    insert or ignore into Temperature_summary
      values (new.day, new.lake_name, new.station, 0, 0);
    update Temperature_summary
    set count_readings_below_75f = count_readings_below_75f + (new.temperature_f < 75),
      count_readings_over_75f = count_readings_over_75f + (new.temperature_f >= 75)
    where new.day = day and new.lake_name = lake_name and new.station = station;
  end;

Answer 2

可以创建触发器以在单个触发器中更新计数器。这可以提高性能并将相关项目收集到一个位置。

create trigger Temperature_count_insert_trigger_lo after insert on Temperature    begin 
    update Temperature_summary set Temperature_summary set count_readings_over_75f = count_readings_over_75f + (new.temperature_f >= 75 ), count_readings_below_75f = count_readings_below_75f + (new.temperature_f < 75)
    where new.day = day and new.lake_name = lake_name and new.station = station;    end;

如果可能发生更新，可能需要在温度表中包含更新的触发处理。

SQLite触发器更新摘要计数

2 个答案: