Data query skills in MySQL

MySQL is currently the most popular open source relational database and is widely used in various web applications and enterprise applications. Data query is one of the most commonly used operations in MySQL, so optimizing query efficiency can greatly improve system performance. . This article will share with you the following commonly used data query techniques in MySQL.

1. Establishing an index
Index is one of the most basic means to improve query efficiency. An index is a data structure that allows the database to locate the required data faster. Common index types include B-Tree, hash table, Bitmap, etc.

To create an index in MySQL, you can use the ALTER TABLE statement. For example, to create an index for the username field of an existing table named users, you can execute the following command:

ALTER TABLE users ADD INDEX (username);

When creating an index, you need Pay attention to the choice of index type. How to choose a more appropriate index type is an issue that needs to be carefully considered.

2. Avoid using SELECT *
When querying data, try to avoid using SELECT , but clearly specify the fields that need to be queried. If there are a large number of fields in the table, querying with SELECT will waste a lot of time and system resources. Therefore, it is recommended to query only the required fields, which can reduce the amount of data transmission and improve query speed.

3. Use EXPLAIN to view the query plan
Use EXPLAIN to view the index and execution method selected by MySQL when executing the query. By viewing the query plan, we can help us optimize the query statement. Usually, the following methods can be used during query optimization:

1. Use joint index
The joint index in MySQL is an index method that can use multiple fields at the same time to optimize queries. Such as the following query:

SELECT * FROM orders WHERE customer_id = 1 AND order_date BETWEEN '2021-01-01' AND '2021-01-31';

can be passed in customer_id and order_date Create a joint index on two columns to improve query speed.

2. Use subquery
Subquery is a way to nest other query statements in the query to filter data and obtain certain data. When using subqueries, you need to pay attention to the execution efficiency of the subquery statements.

The following is an example of using a subquery:

SELECT * FROM orders WHERE customer_id IN (SELECT id FROM customers WHERE age > 18);

In the above query statement , will query the user IDs who are older than 18 years old, and then query the order information related to the user based on these IDs.

4. Use optimizer prompts
There are many optimizer prompts in MySQL that can help us optimize query statements. Optimizer prompts are implemented through HINT syntax, including STRAIGHT_JOIN, USE INDEX, IGNORE INDEX, etc.

For example, using the STRAIGHT_JOIN prompt allows the query results to perform join operations in the specified table order instead of performing join operations according to the optimizer's automatic decision. This prevents the optimizer from choosing the wrong execution path and improves query performance.

5. Avoid using HAVING
The HAVING statement can filter the query results, but the difference between HAVING and WHERE is that HAVING is filtered after the data is retrieved, while WHERE is filtered after the data is retrieved. Data filtering is performed during the process.

Therefore, due to performance considerations, it is recommended that when using HAVING, conditional filtering should be carried out in the WHERE statement as much as possible to reduce the amount of retrieved data and improve query efficiency.

Summary
The above are the data query skills in MySQL. In actual development, in addition to the above methods, there are many other optimization methods that can be used. However, it should be noted that in the process of optimizing queries, a balance needs to be considered between system performance and query efficiency. Only by finding a suitable solution can we not only improve the query efficiency, but also ensure the stable and efficient operation of the system.

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