MySQL - make "Update" a permanent change to a table? - mysql

Question

So I have two tables - "Horses" and "Results". "Horses" lists a bunch of information about each horse, including a spot called "LTE", which totals from an "Earnings" field from the "Results" table. "Results" listed all the results of recent horse shows. I use the following code to calculate LTE -
UPDATE horses
SET horses.LTE = ( SELECT SUM(results.earnings)
FROM results WHERE horses.hname=results.hname )
Which works wonderfully - it updates the LTE column. However...I have to run this code EVERY time I add new data to the "Results" table. I will be adding data month for...well, pretty much ever.
I don't want to have to run this code every time. Is there a way to make the code "permanent," in a sense that the LTE field KNOWS it just calculates whenever new information is added? Or does MySQL not work this way?
Here's a peek at my tables with some data in them.

Answer 1

MySQL doesn't work that way. But you can achieve such functionality by using triggers. For example, you can update your sum each time there's insert in respective table (and/or update), this way you will always have your sum 'cached' and you can recalculate if needed.
If you want a bit more analytics on sum changes, you can follow this pattern, I think it will be helpful

Answer 2

You can use a TRIGGER on INSERT, UPDATE and DELETE to update your table:
-- trigger for INSERT (new rows on table result).
DELIMITER |
CREATE TRIGGER ins_result AFTER INSERT ON results
FOR EACH ROW
BEGIN
UPDATE horses SET horses.LTE = (
SELECT SUM(results.earnings) FROM results WHERE horses.hname = results.hname
);
END;
|
-- trigger for UPDATE (changed rows on table result).
CREATE TRIGGER upd_result AFTER UPDATE ON results
FOR EACH ROW
BEGIN
UPDATE horses SET horses.LTE = (
SELECT SUM(results.earnings) FROM results WHERE horses.hname = results.hname
);
END;
|
-- trigger for DELETE (removed rows on table result).
CREATE TRIGGER del_result AFTER DELETE ON results
FOR EACH ROW
BEGIN
UPDATE horses SET horses.LTE = (
SELECT SUM(results.earnings) FROM results WHERE horses.hname = results.hname
);
END;
|
Another solution could be a VIEW:
CREATE VIEW v_horses AS
SELECT h.*, SUM(r.earnings) AS 'LTE'
FROM horses h INNER JOIN results r ON h.hname = r.hname
GROUP BY h.hname;
With the created VIEW you can get the information about the horses with the following query:
SELECT * FROM v_horses;
In your case you doesn't use a DECIMAL column. So you have to convert the VARCHAR column to SUM the earnings. So in your case you have to use the following VIEW:
CREATE VIEW v_horses AS
SELECT h.*, SUM(CAST(REPLACE(REPLACE(r.earnings, ',', ''), '$', '') AS DECIMAL)) AS 'LTE'
FROM horses h INNER JOIN results r ON h.hname = r.hname
GROUP BY h.hname;
-- for a specific year (like 2017)
CREATE VIEW v_horses AS
SELECT h.*, SUM(CAST(REPLACE(REPLACE(r.earnings, ',', ''), '$', '') AS DECIMAL)) AS 'LTE'
FROM horses h INNER JOIN results r ON h.hname = r.hname
WHERE DATE_FORMAT(STR_TO_DATE(r.`Date`, '%c/%e/%y'), '%Y') = 2017
GROUP BY h.hname;
-- grouped by year (so you can use WHERE on the VIEW):
CREATE VIEW v_horses AS
SELECT h.*, SUM(CAST(REPLACE(REPLACE(r.earnings, ',', ''), '$', '') AS DECIMAL)) AS 'LTE',
DATE_FORMAT(STR_TO_DATE(r.`Date`, '%c/%e/%y'), '%Y') AS 'year'
FROM horses h INNER JOIN results r ON h.hname = r.hname
GROUP BY h.hname, DATE_FORMAT(STR_TO_DATE(r.`Date`, '%c/%e/%y'), '%Y');
Note: In case of using this VIEW you have to remove the column LTE from table horses.