MySQL Generated Columns

How it works

A generated column calculates its value from an allowed expression. It can be virtual or stored, with different storage and indexing implications. This example adds a stored stock_value derived from quantity and unit_cost. Clients update the source fields rather than maintaining a second total manually. A generated value remains a row-level calculation; it is not a place to aggregate other rows or call arbitrary external logic.

Run in a disposable MySQL lab

Use Oracle MySQL 8.4 with InnoDB, not Bhylu’s SQLite playground. The examples were reviewed on MySQL 8.4.8. XAMPP may provide MariaDB, which has different feature and collation behaviour. Check your actual server before running the script. These lessons do not require a change to Bhylu’s PHP, Apache or normal database service.

1. On a separate lab instance, create the empty bhylu_mysql_lab schema. Stop if it already exists; do not overwrite an existing database.
2. Select that schema and import the complete repair-parts seed once. It sets the documented session mode, utf8mb4 collation, UTC and English date-name locale.
3. Run this lesson in a fresh copy of the seed, not after another lesson’s writes. Use the command-line client or a compatible editor. DELIMITER lines are client instructions for stored-program examples.

Download this complete lesson script. Its setup is the linked seed; review the dataset before changing it. Administrative and destructive examples are restricted to the specifically named disposable objects. Every script repeats the session settings because each new connection has its own session. No MySQL browser runner is currently available.

Example

SET NAMES utf8mb4 COLLATE utf8mb4_0900_ai_ci;
SET SESSION sql_mode = 'STRICT_TRANS_TABLES,ONLY_FULL_GROUP_BY,ERROR_FOR_DIVISION_BY_ZERO,NO_ENGINE_SUBSTITUTION';
SET SESSION time_zone = '+00:00';
SET SESSION lc_time_names = 'en_US';
ALTER TABLE parts ADD COLUMN stock_value DECIMAL(12,2)
  GENERATED ALWAYS AS (quantity * unit_cost) STORED;
SELECT id, stock_value FROM parts ORDER BY id;

Expected result

1 | 30
2 | 24
3 | 0
4 | 22.5

Each derived value agrees with the earlier SELECT calculation. Choose a sufficiently wide DECIMAL result type for the domain’s maximum quantity and price. Generated columns can help provide a consistent expression and support selected indexes, but they also become part of the schema compatibility contract. Review changes to their source fields and expression together.