MySQL Subqueries

How it works

A subquery can supply a value to another query. A scalar subquery must return at most one row and one column; an empty scalar result becomes NULL. An aggregate such as AVG without grouping produces a single value, making it suitable here. The example finds parts priced above the average unit cost. It compares the cost of each kind of part, not a stock-weighted average across physical pieces.

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 id, name
FROM parts
WHERE unit_cost > (SELECT AVG(unit_cost) FROM parts)
ORDER BY id;

Expected result

2 | Brass hinge
4 | Wood knob

The average is 3.3625, so the hinge and knob pass. A weighted average would need quantity and a deliberate zero-total policy. Keep that business distinction separate from the mechanics of a subquery. Correlated subqueries refer to the outer row and can be useful, but their actual execution strategy should be checked with a plan rather than assumed from how the text is nested.