mysql tutorial on subqueries vs joins
Jun 29, 2025 am 12:58 AM
The applicable scenarios of subquery and connection are different, and performance depends on the specific situation. 1. Subqueries are suitable for queries with clear logic and simple structure, especially in single-value matching or aggregation scenarios; 2. Connections are more suitable for scenarios where multiple rows of result matching or multiple table fields are obtained simultaneously, and are usually more efficient when processing large amounts of data; 3. In actual use, indexes, query structures and requirements should be combined, and execution plans should be analyzed through EXPLAIN to optimize performance.
If you are just starting to get involved in MySQL query optimization, you may be confused with a question: Is it better to use subqueries or joins? In fact, there is no absolute answer. The key is to understand their respective applicable scenarios and performance performance.
What are subqueries and connections?
A subquery is to nest another query in a SELECT, FROM, or WHERE clause. It is usually used to get data from one table and then search in another table based on this data.
Joining combines data by associating columns of two or more tables. The most common ones are INNER JOIN and LEFT JOIN.
To give a simple example:
-- Subquery writing SELECT name FROM users WHERE id IN (SELECT user_id FROM orders WHERE amount > 100); -- JOIN Writing SELECT u.name FROM users u JOIN orders o ON u.id = o.user_id WHERE o.amount > 100;
The results of these two statements may be the same, but they are executed in different ways.
Which writing method is more efficient?
This problem is very common. Many people think that JOIN must be faster than subquery, but it is not necessarily true. MySQL will automatically optimize subqueries in some cases, converting them into JOIN.
But generally speaking:
- If you only need a small amount of data and subqueries can use indexes, then subqueries are not very efficient.
- If you are dealing with large amounts of data, or need multi-layer nested queries, JOIN is often more efficient because it is designed specifically to merge data sets.
Some practical experience:
- Subqueries may be faster when single values ??match.
- JOIN is more suitable when multiple row results match.
- Using EXISTS to replace IN may have performance improvements, especially on large tables.
When should I use sub-query?
Subqueries are more suitable for queries with clear logic and simple structure. For example, if you want to find users whose order amount is greater than the average, it is very intuitive to use sub-queries:
SELECT name FROM users
WHERE id IN (
SELECT user_id FROM orders
WHERE amount > (SELECT AVG(amount) FROM orders)
);Although this writing method can also be implemented with JOIN, the structure will be much more complicated. So when readability is more important, subqueries are a good choice.
In addition, subquery is also very convenient in some aggregation scenarios. For example, if you want to check the latest order information of each user, you can use subquery to cooperate with GROUP BY:
SELECT u.name, (
SELECT o.amount
FROM orders o
WHERE o.user_id = u.id
ORDER BY o.created_at DESC
LIMIT 1
) AS last_order_amount
FROM users u;When should JOIN be prioritized?
JOIN is more suitable when you need to get fields in multiple tables at the same time, or if you need to reference data from other tables multiple times.
For example, counting the order quantity and total amount of each user:
SELECT u.name, COUNT(o.id) AS order_count, SUM(o.amount) AS total_amount FROM users u LEFT JOIN orders o ON u.id = o.user_id GROUP BY u.id;
This writing method is much simpler and easier to maintain than using subqueries. Moreover, LEFT JOIN can retain users without orders, which is difficult to implement in subquery.
Tips: Don't forget to see the execution plan of EXPLAIN
Whether writing subqueries or JOINs, you must develop the habit of using EXPLAIN to analyze execution plans. Check whether there are indexes and full table scans, so that you can know whether your query is fast or not.
You can add a few common conditions to try:
- Add index to user_id
- Change the subquery to EXISTS form
- Change the JOIN type (such as LEFT vs INNER)
Basically that's it. Master the differences and usage scenarios between the two. The written SQL can not only run fast, but also be easy to understand and maintain.
The above is the detailed content of mysql tutorial on subqueries vs joins. For more information, please follow other related articles on the PHP Chinese website!
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