Analysis of the underlying implementation principle of MySQL INNER JOIN

Overview

In MySQL database, INNER JOIN is a commonly used query operation that matches rows in two or more tables through specified columns to obtain a result set that meets the criteria. This article will explore the underlying implementation principles of INNER JOIN in depth to help readers better understand the working mechanism of JOIN operations.

INNER JOIN syntax

The syntax of INNER JOIN is as follows:

SELECT List name FROM surface1 INNER JOIN surface2 ON surface1.List = surface2.List;

The underlying working principle of INNER JOIN

The underlying implementation principle of INNER JOIN is completed in two steps: filtering and connecting.

• Filtering First, the database engine will filter each table according to the conditions in the INNER JOIN statement, and eliminate rows that do not meet the conditions to reduce the amount of data for subsequent connection operations.
• Joining Next, the database engine will join the filtered two tables according to the connection conditions. There are many specific connection algorithms, including Nested Loop Join, Hash Join and Merge Join.
  • Nested Loop Join (Nested Loop Join) Nested Loop Join is the simplest connection algorithm, which implements connection operations through nested loops. For each row in Table 1, it is compared with Table 2, and if the join condition is met, the two rows are merged into one row and added to the result set.
  • Hash Join (hash join) The Hash Join algorithm uses a hash table to implement join operations. It first takes the join column of one of the tables as the key to the hash table, and then iterates over the other table to find the matching rows by hash matching.
  • Merge Join (Merge Join) The Merge Join algorithm requires that the input two tables have been sorted by the join column. It finds matching rows by traversing the two ordered tables simultaneously and comparing between the two.

Application example

Suppose we have two tables: Table A and Table B, and their structure is as follows:

Table B:

Execute the following INNER JOIN query statement:

SELECT ,  FROM surfaceA A INNER JOIN surfaceB B ON  = ;

According to the Nested Loop Join algorithm, the query execution process is as follows:

• Traverse table A and take out the first line (id=1, name=Alice).
• Traversal in table B and find the matching row (id=1, city=Beijing).
• Add the matching rows to the result set.
• Continue to traverse table B until all matching rows are found.
• Move to the next row in Table A and repeat the above steps.
• The final result set is:

Through this example, we can see that the INNER JOIN operation connects table A and table B according to the id column, and filters out rows that meet the conditions, and finally generates a result set containing the name and city columns.

Summarize

Through this article, we have a deeper understanding of the underlying implementation principles of INNER JOIN in MySQL. The work of INNER JOIN is divided into two steps: filtering and connecting. Various algorithms can be used when connecting, such as Nested Loop Join, Hash Join and Merge Join. Understanding the underlying implementation principles of INNER JOIN helps optimize query performance and improve database efficiency.

When actually using INNER JOIN, we should choose the appropriate connection algorithm according to the specific situation and pay attention to optimizing the writing of query statements to achieve better performance and accurate results.

By deeply studying the underlying implementation of INNER JOIN, we have a more comprehensive understanding of the operating mechanism of MySQL database, providing more ideas and methods for data processing and performance optimization.

This is the end of this article about the underlying implementation principle of MySQL INNER JOIN. For more related content on the underlying principles of MySQL INNER JOIN, please search for my previous articles or continue browsing the related articles below. I hope everyone will support me in the future!