What is a 'temporary table' in databases and what is it used for?

A Temporary Table is a table in a database that exists only during a user session or a transaction. After the session or transaction ends, the temporary table and its data are automatically deleted.

Used for:

• Storing intermediate results of complex queries: Instead of using nested subqueries or CTEs for each part of a query, temporary tables can be created to store intermediate data, often improving query performance and readability.
• Simplifying complex data modification operations: Data requiring multi-step processing can be inserted into a temporary table, necessary modifications performed, and then data from the temporary table used to update or insert into permanent tables.
• Scope limitations of data: Temporary tables allow working with a subset of data without affecting permanent tables or creating conflicts with other users working with the same data.
• Testing and debugging: They enable experimenting with data and queries without risking changes or damage to permanent data.

Example of creating a temporary table in SQL (syntax may vary depending on the DBMS):

-- Creating a temporary table in PostgreSQL
CREATE TEMP TABLE temp_sales AS
SELECT
    product_id,
    SUM(amount) AS total_sales
FROM
    sales
WHERE
    sale_date >= '2023-01-01'
GROUP BY
    product_id;

-- Using the temporary table
SELECT
    p.product_name,
    ts.total_sales
FROM
    products p
JOIN
    temp_sales ts ON p.product_id = ts.product_id;