MySQL is a commonly used relational database management system that is widely used in various website and application development.
Querying data in MySQL is a basic and important operation. This article will introduce MySQL query statements and their usage, helping readers easily master MySQL query skills and methods.
1. SELECT statement
The SELECT statement is the most commonly used query statement in MySQL. It can select data from one or more tables and return a result set. The syntax structure of the SELECT statement is as follows:
SELECT *|column1,column2,... FROM table1,table2,... WHERE condition;
where:
- *: Indicates selecting all columns;
- column1,column2,...: Indicates selecting certain columns;
- table1,table2,...: Indicates which tables to select data from;
- WHERE condition: Indicates filtering conditions.
2. Basic query
- Query all data
SELECT * FROM table_name;
This statement will Returns all the data in the table, but it is not recommended to be used in actual applications because if the amount of data in the table is very large, the efficiency of the query may be reduced.
- Query the data of certain columns
SELECT column1,column2,...FROM table_name;
This statement can select certain columns in the table Querying the data in the column can appropriately improve query efficiency.
- Query data that meets the condition
SELECT * FROM table_name WHERE condition;
This statement will return all data that meets the condition, where condition is Query conditions. For example:
SELECT * FROM students WHERE gender="male";
This statement will return information about all students whose gender is male.
- Fuzzy query
SELECT * FROM table_name WHERE column_name LIKE "keyword";
This statement can be used for fuzzy query, where keyword is the keyword . For example:
SELECT * FROM students WHERE name LIKE "%李%";
This statement will return the information of all students whose names contain the word "李".
- Range query
SELECT * FROM table_name WHERE column_name BETWEEN value1 AND value2;
This statement can perform range query, in which both value1 and value2 is the value of a certain column. For example:
SELECT * FROM students WHERE age BETWEEN 18 AND 22;
This statement will return information about all students between the ages of 18 and 22.
3. Advanced query
- Sub query
SELECT * FROM table_name WHERE column_name=(SELECT column_name FROM table_name WHERE condition);
Subquery is a query that contains another query. It is often used to query the results under a certain condition.
For example:
SELECT * FROM students WHERE age=(SELECT MAX(age) FROM students);
This statement will return the information of the oldest student.
- Connection query
Connection query refers to obtaining more complete data by connecting multiple tables. Commonly used connection methods include inner join, left join, right join and Fully connected.
(1) Inner join query
SELECT * FROM table1 INNER JOIN table2 ON condition;
The condition after ON indicates the condition when two tables are connected.
(2) Left join query
SELECT * FROM table1 LEFT JOIN table2 ON condition;
The left join refers to based on the left table, the left table Keep all the data, and add the data that meets the conditions in the right table to the left table.
(3) Right join query
SELECT * FROM table1 RIGHT JOIN table2 ON condition;
Right join means based on the right table, the Keep all the data, and add the data that meets the conditions in the left table to the right table.
(4) Full join query
SELECT * FROM table1 FULL OUTER JOIN table2 ON condition;
Full join query means to retain all the data in the left and right tables , and match data that meets the conditions together.
- Group query
SELECT column_name1,column_name2,...FROM table_name GROUP BY column_name;
Group query refers to sorting the table according to a certain column Group display is often used to count the number of a certain field or calculate the average.
For example:
SELECT gender,COUNT(*) FROM students GROUP BY gender;
This statement will return the number of male and female students.
- Sort query
SELECT * FROM table_name ORDER BY column_name;
Sort query is to sort by the value of a certain column, which can be done by ASC (ascending order) ) or DESC (descending order) to specify the sorting method.
For example:
SELECT * FROM students ORDER BY age DESC;
This statement will sort the information of all students in descending order of age.
4. Summary
The above are the basic and advanced usage methods of MySQL query statements. Through the introduction of this article, I believe that readers have a deeper understanding and mastery of MySQL query statements. .
In practical applications, the query method should be reasonably selected according to the actual needs of the query to improve query efficiency and accuracy of results. At the same time, when using MySQL, you must also pay attention to protecting data security to avoid accidentally deleting or leaking important data.
The above is the detailed content of mysql how to query. For more information, please follow other related articles on the PHP Chinese website!

How to effectively monitor MySQL performance? Use tools such as mysqladmin, SHOWGLOBALSTATUS, PerconaMonitoring and Management (PMM), and MySQL EnterpriseMonitor. 1. Use mysqladmin to view the number of connections. 2. Use SHOWGLOBALSTATUS to view the query number. 3.PMM provides detailed performance data and graphical interface. 4.MySQLEnterpriseMonitor provides rich monitoring functions and alarm mechanisms.

The difference between MySQL and SQLServer is: 1) MySQL is open source and suitable for web and embedded systems, 2) SQLServer is a commercial product of Microsoft and is suitable for enterprise-level applications. There are significant differences between the two in storage engine, performance optimization and application scenarios. When choosing, you need to consider project size and future scalability.

In enterprise-level application scenarios that require high availability, advanced security and good integration, SQLServer should be chosen instead of MySQL. 1) SQLServer provides enterprise-level features such as high availability and advanced security. 2) It is closely integrated with Microsoft ecosystems such as VisualStudio and PowerBI. 3) SQLServer performs excellent in performance optimization and supports memory-optimized tables and column storage indexes.

MySQLmanagescharactersetsandcollationsbyusingUTF-8asthedefault,allowingconfigurationatdatabase,table,andcolumnlevels,andrequiringcarefulalignmenttoavoidmismatches.1)Setdefaultcharactersetandcollationforadatabase.2)Configurecharactersetandcollationfor

A MySQL trigger is an automatically executed stored procedure associated with a table that is used to perform a series of operations when a specific data operation is performed. 1) Trigger definition and function: used for data verification, logging, etc. 2) Working principle: It is divided into BEFORE and AFTER, and supports row-level triggering. 3) Example of use: Can be used to record salary changes or update inventory. 4) Debugging skills: Use SHOWTRIGGERS and SHOWCREATETRIGGER commands. 5) Performance optimization: Avoid complex operations, use indexes, and manage transactions.

The steps to create and manage user accounts in MySQL are as follows: 1. Create a user: Use CREATEUSER'newuser'@'localhost'IDENTIFIEDBY'password'; 2. Assign permissions: Use GRANTSELECT, INSERT, UPDATEONmydatabase.TO'newuser'@'localhost'; 3. Fix permission error: Use REVOKEALLPRIVILEGESONmydatabase.FROM'newuser'@'localhost'; then reassign permissions; 4. Optimization permissions: Use SHOWGRA

MySQL is suitable for rapid development and small and medium-sized applications, while Oracle is suitable for large enterprises and high availability needs. 1) MySQL is open source and easy to use, suitable for web applications and small and medium-sized enterprises. 2) Oracle is powerful and suitable for large enterprises and government agencies. 3) MySQL supports a variety of storage engines, and Oracle provides rich enterprise-level functions.

The disadvantages of MySQL compared to other relational databases include: 1. Performance issues: You may encounter bottlenecks when processing large-scale data, and PostgreSQL performs better in complex queries and big data processing. 2. Scalability: The horizontal scaling ability is not as good as Google Spanner and Amazon Aurora. 3. Functional limitations: Not as good as PostgreSQL and Oracle in advanced functions, some functions require more custom code and maintenance.


Hot AI Tools

Undresser.AI Undress
AI-powered app for creating realistic nude photos

AI Clothes Remover
Online AI tool for removing clothes from photos.

Undress AI Tool
Undress images for free

Clothoff.io
AI clothes remover

Video Face Swap
Swap faces in any video effortlessly with our completely free AI face swap tool!

Hot Article

Hot Tools

SublimeText3 Mac version
God-level code editing software (SublimeText3)

Safe Exam Browser
Safe Exam Browser is a secure browser environment for taking online exams securely. This software turns any computer into a secure workstation. It controls access to any utility and prevents students from using unauthorized resources.

Atom editor mac version download
The most popular open source editor

EditPlus Chinese cracked version
Small size, syntax highlighting, does not support code prompt function

SecLists
SecLists is the ultimate security tester's companion. It is a collection of various types of lists that are frequently used during security assessments, all in one place. SecLists helps make security testing more efficient and productive by conveniently providing all the lists a security tester might need. List types include usernames, passwords, URLs, fuzzing payloads, sensitive data patterns, web shells, and more. The tester can simply pull this repository onto a new test machine and he will have access to every type of list he needs.