Use trigger to populate another column I have two counters in a mysql table. I would like to have another column that contains the difference between counter column 1 and counter column 2. Could I use a trigger after counter column 2 is updated which will subtract the number in counter 2 from the number in counter 1 and put the results in count column 3? I would want to index count column 3 for queries.
You can use a generated column in your CREATE TABLE statement to always have an up-to-date value:

CREATE TABLE TestTable(
    id INT AUTO_INCREMENT PRIMARY KEY,
    col1 INT NOT NULL,
    col2 INT NOT NULL,
    col3 INT AS (col1 - col2)
);


The column can be virtual or stored (virtual is default) and each has their own use cases as detailed in the link above. 

Note: only applicable for MySQL 5.7.6 &amp; higher.