mysql query connection

Introduction to MySQL Query Connection

In database management, query connection is a very important part. Join refers to the process of joining two or more tables based on the relationship between them to complete data query. In MySQL, we can implement query connections by using the JOIN statement. This article will provide an in-depth introduction to the use of MySQL query connections.

Classification of MySQL query connections

MySQL query connections are mainly divided into three categories: inner joins, outer joins and cross joins.

1. INNER JOIN

Inner join means that there is a corresponding relationship between records in two or more tables. This relationship can be represented by primary keys and foreign keys. . Inner joins return results by retaining the same records. That is, query results will be returned only if matching records exist in both tables.

The syntax of inner join is as follows:

SELECT col1, col2, ...
FROM table1
INNER JOIN table2
ON table1.column1 = table2.column2;

Among them, col1, col2, ... are the columns to be returned; table1 and table2 are the tables to be queried, and column1 and column2 are the columns that connect the tables.

2. OUTER JOIN

Outer joins are divided into left joins and right joins. This join refers to when connecting two or more tables, including All records are retrieved, even if one table has no matching records from the other table.

Left join: Left join means that the left table fetches all records, while the right table fetches only matching records. If there are no matching records between the left table and the right table, the right table will return a NULL value.

The syntax of left join is as follows:

SELECT col1, col2, ...
FROM table1
LEFT JOIN table2
ON table1.column1 = table2.column2;

Right join: Right join is similar to left join, except that the roles of the left table and the right table are reversed.

The syntax of right join is as follows:

SELECT col1, col2, ...
FROM table1
RIGHT JOIN table2
ON table1.column1 = table2.column2;

Note: In MySQL, there is also a FULL OUTER JOIN (full outer join), but MySQL does not support it. Therefore, we can simulate a full outer join through the UNION statement.

3. Cross join (CROSS JOIN)

Cross join refers to a join without any join conditions between two or more tables. Simply put, cross joins allow you to query all possible combinations.

The syntax of cross join is as follows:

SELECT col1, col2, ...
FROM table1
CROSS JOIN table2;

Among them, col1, col2, ... is the column to be returned; table1 and table2 are the tables to be queried.

Application examples of MySQL query connection

Now let us understand the use of MySQL query connection through some practical examples.

Example 1: Inner join

Suppose we have two tables student and course. Their columns and contents are as follows:

student table:

##1Jack182Tom193Jerry204Bob18

id	name	age

course table:

course_idcourse_namestudent_id1Math12English23Science24History3Now we want to query the courses selected by each student, we can use the following SQL statement:

SELECT student.name, course.course_name

FROM student
INNER JOIN course
ON student.id = course.student_id;

This query will return the following results:

namecourse_nameJackMathTomEnglishTomScienceJerryHistoryExample 2: Left Join

Still taking the above student table and course table as an example, if we want to query the courses chosen by each student, including those students who have not chosen a course. You can use the following SQL statement:

SELECT student.name, course.course_name

FROM student
LEFT JOIN course
ON student.id = course.student_id;

This query will return the following results:

namecourse_nameJackMathTom#EnglishTomScienceJerryHistoryBobNULLExample 3: Right Join

Continue to use the above student table and course table as an example, if we want to query the students of each course, including students who have not chosen the course. You can use the following SQL statement:

SELECT student.name, course.course_name

FROM student
RIGHT JOIN course
ON student.id = course.student_id;

This query will return the following results:

namecourse_nameJackMathTom#EnglishTomScienceJerryHistoryNULLGeography

Example 4: Cross connection

Cross connection allows you to query all possible combinations. For example, we have a table containing three elements A, B and C:

table1:

##1A2B3C

id	name

We can use the following SQL statements to perform cross connections:

SELECT t1.name, t2.name

FROM table1 t1
CROSS JOIN table1 t2;

This query will Returns the following results:

namenameAAABACBAB#B##BC##CBCConclusionMySQL query connection is an important part of the SQL language and can help us query data containing relationships. In this article, we introduce the syntax and application examples of the four types of joins: inner join, left join, right join and cross join. Hope this article helps you use MySQL query connection.

C
A
	##C

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