MySQL Temporary Tables

Summary: in this tutorial, we will discuss MySQL temporary tables and show you how to create, use, and drop temporary tables.

Introduction to MySQL temporary tables

In MySQL, a temporary table is a special type of table that allows you to store a temporary result set, which you can reuse several times in a single session.

A temporary table is handy when it is impossible or expensive to query data that requires a single SELECT statement. In such cases, you can use a temporary table to store the immediate result and use another query to process it.

A MySQL temporary table has the following features:

A temporary table is created by using CREATE TEMPORARY TABLE statement. Notice that the keyword TEMPORARY is added between the CREATE and TABLE keywords.
MySQL removes the temporary table automatically when the session ends or the connection is terminated. Also, you can use the DROP TABLE statement to remove a temporary table explicitly when you are no longer using it.
A temporary table is only available and accessible to the client that creates it. Different clients can create temporary tables with the same name without causing errors because only the client that creates the temporary table can see it. However, in the same session, two temporary tables cannot share the same name.
A temporary table can have the same name as a regular table in a database. For example, if you create a temporary table named employees in the sample database, the existing employees table becomes inaccessible. Every query you issue against the employees table is now referring to the temporary table employees. When you drop the employees temporary table, the regular employees table is available and accessible.

Even though a temporary table can have the same name as a regular table, it is not recommended. Because this may lead to confusion and potentially cause an unexpected data loss.

For example, if the connection to the database server is lost and you reconnect to the server automatically, you cannot differentiate between the temporary table and the regular one.

Then, you may issue a DROP TABLE statement to remove the permanent table instead of the temporary table, which is not expected.

To avoid this issue, you can use the DROP TEMPORARY TABLE statement to drop a temporary table instead of the DROP TABLE statement

MySQL CREATE TEMPORARY TABLE statement

The syntax of the CREATE TEMPORARY TABLE statement is similar to the syntax of the CREATE TABLE statement except for the TEMPORARY keyword:

CREATE TEMPORARY TABLE table_name(
   column1 datatype constraints,
   column1 datatype constraints,
   ...,
   table_constraints
);Code language: SQL (Structured Query Language) (sql)

To create a temporary table whose structure is based on an existing table, you cannot use the CREATE TEMPORARY TABLE ... LIKE statement. Instead, you use the following syntax:

CREATE TEMPORARY TABLE temp_table_name
SELECT * FROM original_table
LIMIT 0;Code language: SQL (Structured Query Language) (sql)

1) Creating a temporary table example

First, create a new temporary table called credits that stores customers’ credits:

CREATE TEMPORARY TABLE credits(
  customerNumber INT PRIMARY KEY, 
  creditLimit DEC(10, 2)
);Code language: SQL (Structured Query Language) (sql)

Then, insert rows from the customers table into the temporary table credits:

INSERT INTO credits(customerNumber, creditLimit)
SELECT 
  customerNumber, 
  creditLimit 
FROM 
  customers 
WHERE 
  creditLimit > 0;Code language: SQL (Structured Query Language) (sql)

2) Creating a temporary table whose structure is based on a query example

The following example creates a temporary table that stores the top 10 customers by revenue. The structure of the temporary table is derived from a SELECT statement:

CREATE TEMPORARY TABLE top_customers
SELECT p.customerNumber, 
       c.customerName, 
       ROUND(SUM(p.amount),2) sales
FROM payments p
INNER JOIN customers c ON c.customerNumber = p.customerNumber
GROUP BY p.customerNumber
ORDER BY sales DESC
LIMIT 10;Code language: SQL (Structured Query Language) (sql)

Now, you can query data from the top_customers temporary table like querying from a permanent table:

SELECT 
    customerNumber, 
    customerName, 
    sales
FROM
    top_customers
ORDER BY sales;Code language: SQL (Structured Query Language) (sql)

Dropping a temporary table

You can use the DROP TABLE statement to remove temporary tables however it is good practice to add the TEMPORARY keyword as follows:

DROP TEMPORARY TABLE table_name;Code language: SQL (Structured Query Language) (sql)

The DROP TEMPORARY TABLE statement removes a temporary table only, not a regular table. It helps you avoid the risk of mistakenly dropping a regular table with the same name as the temporary table.

For example, to remove the top_customers temporary table, you use the following statement:

DROP TEMPORARY TABLE top_customers;Code language: SQL (Structured Query Language) (sql)

Notice that if you try to remove a regular table with the DROP TEMPORARY TABLE statement, you will get an error message saying that the table that you are trying to drop is unknown.

If you develop an application that uses connection pooling or persistent connections, it is not guaranteed that the temporary tables are removed automatically when your application is terminated.

Because the database connection that the application uses may be still open and placed in a connection pool for other clients to reuse later.

Therefore, it is a good practice to always remove the temporary tables whenever you are no longer use them.

Checking if a temporary table exists

MySQL does not provide a function or statement to directly check if a temporary table exists.

However, you can create a stored procedure that checks if a temporary table exists or not as follows:

DELIMITER //
CREATE PROCEDURE check_table_exists(table_name VARCHAR(100)) 
BEGIN
    DECLARE CONTINUE HANDLER FOR SQLSTATE '42S02' SET @err = 1;
    SET @err = 0;
    SET @table_name = table_name;
    SET @sql_query = CONCAT('SELECT 1 FROM ',@table_name);
    PREPARE stmt1 FROM @sql_query;
    IF (@err = 1) THEN
        SET @table_exists = 0;
    ELSE
        SET @table_exists = 1;
        DEALLOCATE PREPARE stmt1;
    END IF;
END //
DELIMITER ;Code language: SQL (Structured Query Language) (sql)

In this procedure, we try to select data from a temporary table. If the temporary table exists, the @table_exists variable is set to 1, otherwise, it is set to 0.

This statement calls the check_table_exists stored procedure to check if the temporary table credits exists:

CALL check_table_exists('credits');
SELECT @table_exists;Code language: SQL (Structured Query Language) (sql)

Here is the output:

Summary

MySQL automatically deletes all temporary tables once the session is ended.
Use the CREATE TEMPORARY TABLE statement to create a temporary table.
Use the DROP TEMPORARY TABLE statement to drop a temporary table.

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