用于更新摘要计数的 SQLite 触发器 [英] SQLite trigger to update Summary Counts

问题描述

考虑以下两个(假设的)表格

Consider the following two (hypothetical) tables

温度

* day
* time
* lake_name
* station
* temperature_f

Temperature_summary

Temperature_summary

* day
* lake_name
* station
* count_readings_over_75f
* count_readings_below_75f

如何编写 SQLite 触发器来更新插入时的 temperature_summary 表.我想增加计数.

How can I write an SQLite Trigger to update the temperature_summary table on insert. I want to increment the count.

谢谢，杰夫

推荐答案

这假设您在插入当天的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.

This assumes you have already created the record for the day/lake_name/station before inserting temperatures on that day. Of course, you could add another trigger to do that.

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

You can combine these into one slightly more complex trigger

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;

确保 Temperature_summary 中有一行需要更新 (a) 在 Temperature_summary 的 (day,lake_name,station) 上创建唯一索引，或将这些列设为主键，以及 (b)在触发器中执行插入或忽略，如下所示:

To insure that there is a row in Temperature_summary to update (a) make a unique index on Temperature_summary's (day, lake_name, station), or make those columns the primary key, and (b) do an insert or ignore in the trigger like so:

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;

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