Column calculated from another column?

Built-in powers of generated columns

With MySQL versions 5.7 and above, you can unlock the capabilities of generated columns. Slide into the choice between virtual and stored types based on your demands for speed and storage:

• Virtual: Choose this when you're okay with billing some extra computational time for fresh data.
• Stored: When the speed of the read operation is the finish line of your race, and you've got storage to spare, opt for this.

Remember, these hard-working generated columns could be part of an index, proving especially useful when the determined values are often part of SELECT outputs, WHERE conditions, or in JOIN operations.

Leveraging triggers for automation

If the support for generated columns isn't present in your version of MySQL, or if you're looking for more control in the calculation, put triggers into play. They intercept INSERT and UPDATE events to automatically calculate new column values, ensuring live updates akin to a diligent accountant adjusting books:

DELIMITER |
CREATE TRIGGER before_insert_sales
BEFORE INSERT ON sales
FOR EACH ROW
BEGIN
   SET NEW.total_sale = NEW.price * NEW.quantity;  -- Auto adding sales, much wow!
END;
|
DELIMITER ;
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The symbol NEW refers to the row that's being inserted. Triggers allow room for more complex logic, much like a customized chef's recipe that doesn't fit within a kitchen's routine menu.

Boosting performance with indexing

With performance in your scope, do not forget that indexes on calculated columns are fleet-footed sprinters. If your calculated column is part of a regular query, consdier pinning an index to it:

CREATE INDEX idx_total_sale ON sales (total_sale);  -- Hello speed, goodbye slow!


But remember, speed isn't a free lunch—indexes do rack up costs in terms of disk space and write speed. So, judicious application is the key!

Be vigilant of spirits and traps

Watch out! Automated calculations are a double-edged sword. While ensuring fresh data, they can also spin messy webs. Maintain a relationship between "calculated" and "value" to ensure consistency.

Be keen about updating a generated column directly—it's a big no-no. Updates on base columns automatically ripple through Stored Generated Columns, but direct updates are as welcome as a hawk in a hen house—expect an error.

When handling triggers, stay wary of their execution order and any interactions with other triggers. Think through the system as a whole to prevent your machinery from turning into a house of misbehaving dominoes.