mysql how to query

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

1. 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.

2. 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.

3. 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.

4. 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".

5. 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

1. 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.

2. 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.

3. 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.

4. 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.

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