In-depth understanding of temporary tables in MySQL

The temporary table in MySQL is a special table that can store some temporary data in the MySQL database. Temporary tables are different from ordinary tables in that they do not require users to manually create them in the database and only exist in the current connection and session. This article will take an in-depth look at temporary tables in MySQL.

1. What is a temporary table
A temporary table is a special type of table in MySQL that only exists in the current database session. Temporary tables do not require users to manually create them in the database in advance, but are automatically created when users perform operations such as SELECT, INSERT, UPDATE, and DELETE. Temporary tables can significantly improve the performance of MySQL databases, making queries faster and more efficient.

2. Classification of temporary tables
Temporary tables in MySQL are mainly divided into two types, namely local temporary tables and global temporary tables.

1. Local temporary table:
   Local temporary table only exists in the current database connection and is deleted only at the end of the session that created the table. Local temporary tables are created in the current connection's temporary directory, usually the /tmp or /var/tmp directory.
2. Global temporary table:
   Global temporary table is created in the database server and is visible throughout the server. When the MySQL server is shut down, the global temporary table is automatically deleted. The creation method of global temporary table is similar to that of ordinary table, but special prefix symbols need to be used, such as: ## table name.

3. Usage scenarios of temporary tables

Using temporary tables can help solve some performance problems and improve query efficiency. The following introduces some usage scenarios of temporary tables:

1. Sorting and grouping
Temporary tables can help us perform grouping and sorting operations and solve the sorting performance problem of large amounts of data. When the amount of data is very large, query operations may take a long time to complete because the results need to be sorted or grouped by specific fields, and these operations require a lot of calculations.

Using temporary tables can effectively solve this problem. By inserting the fields that need to be sorted and other necessary information into the temporary table, we can use indexes to quickly sort the data, thereby greatly improving query efficiency.

2. Deep query
Temporary tables are also very useful when we need to perform in-depth queries on data with complex structures. For example, if we need to query a relational database containing multiple tables, we need to use JOIN operations multiple times. In this case, we can use a temporary table to save the results before further operations.

Temporary tables can help us avoid repeated JOIN operations, improve query efficiency, and reduce the load on the database server.

3. Use across multiple connections
Temporary tables can also be used across multiple connections. For example, we need to share the data of a table between multiple servers, but each server needs Perform different query operations according to different needs. At this time, temporary tables are a very good choice.

Temporary tables can store data, and each server can query data by connecting to the temporary table, thus avoiding the trouble and risk of copying data between different servers.

4. Creation, use and deletion of temporary tables

1. Create temporary tables
   Create local temporary tables:

CREATE TEMPORARY TABLE < table_name> (

...

);

Create a global temporary table:

CREATE TEMPORARY TABLE ## (

...

);

2. Use temporary table
   Use temporary table:

SELECT ...
FROM ;

3. Delete temporary Table
   Delete temporary table:

DROP TEMPORARY TABLE ;

In short, temporary table is a very useful MySQL feature, which can significantly improve MySQL Database performance, making queries faster and more efficient. However, when using temporary tables, you need to pay attention to ensure the correctness and integrity of the data.

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