SQLite trigger to update Summary Counts

Question

Consider the following two (hypothetical) tables

Temperature

* day
* time
* lake_name
* station
* temperature_f

Temperature_summary

Answer 1

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;

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;

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;

Answer 2

A trigger can be created to update the counters in a single trigger. This will arguably improve performance and collect the related items into a single location.

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;

It may be necessary to include trigger processing for updates in the Temperature table if these can occur.