MySQL SELECT

How it works

SELECT chooses expressions and columns for the result. FROM identifies the source rows. A useful query states which information a caller needs, rather than returning SELECT * everywhere. Here a volunteer needs part names and stock quantities, not internal notes or every future column. Explicit projection also makes a query contract more stable when a table gains another field. A result column can be a calculation as well as a stored value.

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';
SELECT name, quantity, quantity * unit_cost AS stock_value
FROM parts
ORDER BY id;

Expected result

Canvas patch | 12 | 30
Brass hinge | 4 | 24
Cotton cord | 0 | 0
Wood knob | 6 | 22.5

The multiplication calculates value from the current stock, without storing another total that could become stale. DECIMAL arithmetic preserves the intended exact inputs. The alias names the calculated column for readers and client code. Choosing columns does not filter rows; all four source records still participate. Later WHERE examples narrow the records, and aggregate examples change the level of the result.