Question

Let’s say that I want to have a table that logs the date and the number of columns in some other table (or really any sort of math / string concat etc).

CREATE TABLE `log` ( `id` INTEGER NOT NULL AUTO_INCREMENT , `date` DATETIME NOT NULL , `count` INTEGER NOT NULL , PRIMARY KEY (`id`) ); 

Is it possible to have the count column calculated for me whenever I do an insert?

e.g. do something like:

INSERT INTO log (date='foo'); 

and have count calculated by mysql.

Obviously I could do it myself by doing a query to get the count and inserting it, but this would be better.

Answer 1

Triggers are the best tool for annotating data when a table is changed by insert, update or delete.

To automatically set the date column of a new row in the log with the current date, you’d create a trigger that looked something like this:

create trigger log_date before insert on log  for each row begin    set new.date = current_date() end;