SQL Temporary Tables

Introduction

Temporary tables are a powerful feature in SQL that allow you to store and manipulate intermediate results during the execution of a query or a session. They are similar to regular tables, but they exist only for the duration of a specific session or transaction. Temporary tables provide a flexible way to break down complex queries, manage intermediate data, or store temporary results without polluting the database with permanent tables.

In this article, we will explore what temporary tables are, their types, when and how to use them, and best practices for working with them.

What Are Temporary Tables?

Temporary tables are tables that exist temporarily during a database session or transaction. They are stored in a special database schema and are automatically deleted when the session ends or when explicitly dropped. Because they are temporary, they are not included in permanent schema metadata, making them a useful tool for managing intermediate data without long-term persistence.

There are two main types of temporary tables:

• Local Temporary Tables: These are visible only within the session that created them and are dropped automatically when the session ends.
• Global Temporary Tables: These are visible to all sessions but behave similarly to local temporary tables in that they are deleted when all sessions that are using them end.

Creating Temporary Tables

Creating a temporary table is similar to creating a regular table, with the main difference being the use of the keyword TEMPORARY or TEMP in the CREATE TABLE statement. Most databases, including MySQL, PostgreSQL, and SQL Server, support temporary tables.

Example: Creating a local temporary table in SQL:

CREATE TEMPORARY TABLE temp_employees (
    id INT PRIMARY KEY,
    name VARCHAR(100),
    department VARCHAR(50),
    salary DECIMAL(10, 2)
);

In this example, a temporary table named temp_employees is created. It will only exist for the duration of the session in which it was created.

Types of Temporary Tables

Local Temporary Tables

Local temporary tables are specific to the session that creates them. They are automatically dropped when the session ends or the user manually drops them. In most databases, local temporary tables are prefixed with a single hash (#) in SQL Server or simply created with the TEMPORARY keyword in databases like MySQL and PostgreSQL.

SQL Server Example:

CREATE TABLE #temp_employees (
    id INT PRIMARY KEY,
    name VARCHAR(100),
    department VARCHAR(50),
    salary DECIMAL(10, 2)
);

In SQL Server, the # symbol indicates that the table is a local temporary table, and it will automatically be dropped when the session ends.

Global Temporary Tables

Global temporary tables are accessible to all sessions but behave similarly to local temporary tables in that they are dropped when no sessions are using them. In SQL Server, global temporary tables are prefixed with a double hash (##).

SQL Server Example:

CREATE TABLE ##global_temp_employees (
    id INT PRIMARY KEY,
    name VARCHAR(100),
    department VARCHAR(50),
    salary DECIMAL(10, 2)
);

In this example, the global temporary table ##global_temp_employees can be accessed by any session, but it will be dropped automatically once all active sessions that reference it are closed.

How to Use Temporary Tables

Temporary tables are typically used in scenarios where you need to store intermediate query results or perform complex data transformations. Here are a few common use cases:

1. Breaking Down Complex Queries

Sometimes, you might need to break down complex queries into smaller, more manageable parts. Temporary tables allow you to store intermediate results and use them in subsequent queries, which can simplify logic and improve performance.

Example: Using a temporary table to break down a complex query.

-- Step 1: Create a temporary table to store intermediate results
CREATE TEMPORARY TABLE temp_high_salary AS
SELECT id, name, department, salary
FROM employees
WHERE salary > 50000;

-- Step 2: Use the temporary table in further analysis
SELECT department, AVG(salary) AS avg_salary
FROM temp_high_salary
GROUP BY department;

In this example, we first create a temporary table temp_high_salary to store employees with a salary greater than 50,000. Then, we use this temporary table to calculate the average salary for each department.

2. Performance Optimization

Temporary tables can be used to improve query performance, especially when dealing with large datasets or repeated calculations. Instead of running the same subquery multiple times, you can store the results in a temporary table and reference it as needed.

Example: Storing results in a temporary table to avoid redundant calculations.

-- Step 1: Create a temporary table for frequently used results
CREATE TEMPORARY TABLE temp_sales AS
SELECT product_id, SUM(sales_amount) AS total_sales
FROM sales
GROUP BY product_id;

-- Step 2: Use the temporary table in multiple queries
SELECT product_id, total_sales
FROM temp_sales
WHERE total_sales > 10000;

SELECT product_id, total_sales
FROM temp_sales
WHERE total_sales < 5000;

By creating the temporary table temp_sales, you avoid calculating the total sales for each product multiple times. This can lead to significant performance improvements, particularly when working with large datasets.

3. Storing Intermediate Results in ETL Processes

In Extract, Transform, Load (ETL) processes, temporary tables can be used to store and manipulate intermediate results during the transformation phase. This helps manage complex data transformations while keeping the process organized.

Example: Using temporary tables in an ETL process.

-- Extract data from the source
CREATE TEMPORARY TABLE temp_raw_data AS
SELECT *
FROM source_table
WHERE created_at >= '2024-01-01';

-- Transform the data
UPDATE temp_raw_data
SET status = 'processed'
WHERE status = 'pending';

-- Load the transformed data into the target table
INSERT INTO target_table
SELECT *
FROM temp_raw_data;

In this ETL process, temp_raw_data stores raw data extracted from the source, which is then transformed (i.e., updating the status) before being loaded into the target table.

Best Practices for Using Temporary Tables

While temporary tables can be very useful, there are some best practices to keep in mind to avoid potential pitfalls:

1. Drop Temporary Tables When No Longer Needed
   Although temporary tables are automatically dropped at the end of a session, it is good practice to explicitly drop them when they are no longer needed. This ensures that you free up resources as soon as possible.

   DROP TEMPORARY TABLE IF EXISTS temp_employees;
   

2. Use Temporary Tables Sparingly
   Temporary tables consume system resources such as memory and disk space, so it’s important to use them judiciously. If a query can be accomplished without a temporary table, consider alternative approaches like Common Table Expressions (CTEs).

3. Index Temporary Tables for Performance
   If you are performing complex queries or joining temporary tables with other tables, consider adding indexes to temporary tables to improve performance.

   CREATE INDEX idx_temp_employees_salary ON temp_employees(salary);
   

4. Be Aware of Table Scope
   Remember that local temporary tables are only visible to the session that creates them, whereas global temporary tables are accessible to all sessions. Choose the appropriate type based on your requirements.

Conclusion

Temporary tables are an essential tool in SQL, providing a convenient way to store and manipulate intermediate results without creating permanent database objects. They are particularly useful for breaking down complex queries, improving performance, and handling intermediate results in ETL processes. By following best practices and understanding their limitations, temporary tables can greatly enhance the efficiency and manageability of your SQL queries.