Temporary Tables in MySql

Create a Temporary Table: Use the CREATE TEMPORARY TABLE statement to create a temporary table. Temporary tables are session-specific and are automatically dropped when the session ends or when they are explicitly dropped.

Drop the Temporary Table (Optional): Temporary tables are automatically dropped when the session ends. However, if you want to explicitly drop the temporary table within the session, use the DROP TEMPORARY TABLE statement.

Example:

CREATE TEMPORARY TABLE temp_table (

    id INT AUTO_INCREMENT PRIMARY KEY,

    name VARCHAR(255)

);

INSERT INTO temp_table (name) VALUES ('John'), ('Alice'), ('Bob');

SELECT * FROM temp_table WHERE name = 'Alice';

DROP TEMPORARY TABLE temp_table;

Remember, temporary tables are useful for storing intermediate results within a session and are automatically cleaned up when the session ends, making them suitable for temporary data storage needs.

Using intermediate tables in SQL queries is a common practice for breaking down complex operations into smaller, manageable steps. Here are a few examples of how you can use intermediate tables to answer queries:

Example 1: Using a Common Table Expression (CTE)

-- Create an intermediate table (CTE) to calculate average salary by department

WITH AvgSalaryByDept AS (

    SELECT department_id, AVG(salary) AS avg_salary

    FROM employees

    GROUP BY department_id

)

-- Query to find departments where average salary is above a certain threshold

SELECT department_id

FROM AvgSalaryByDept

WHERE avg_salary > 50000;

In this example:

• We first calculate the average salary by department using a Common Table Expression (WITH clause).
• Then, we query the CTE to find departments where the average salary is above $50,000.

Example 2: Using Temporary Tables

-- Create a temporary table to store employees hired in the last year

CREATE TEMPORARY TABLE recent_hires AS (

    SELECT employee_id, hire_date

    FROM employees

    WHERE hire_date >= CURDATE() - INTERVAL 1 YEAR

);

-- Query to find the average tenure of recent hires

SELECT AVG(DATEDIFF(CURDATE(), hire_date)) AS avg_tenure_in_days

FROM recent_hires;

-- Drop the temporary table (optional)

DROP TEMPORARY TABLE recent_hires;

In this example:

• We create a temporary table recent_hires to store employees hired in the last year.
• We then query this temporary table to find the average tenure of these recent hires.
• Finally, we drop the temporary table once we are done with it.

Example 3: Using Subqueries

-- Query to find employees whose salary is above the department average

SELECT employee_id, salary

FROM employees e

WHERE salary > (

    SELECT AVG(salary)

    FROM employees

    WHERE department_id = e.department_id

);

In this example:

• We use a subquery to calculate the average salary for each department.
• The main query then selects employees whose salary is higher than their department's average salary.

Benefits of Using Intermediate Tables:

• Modularity: Breaking down complex queries into smaller, more understandable steps.
• Performance: In some cases, using intermediate results can optimize query execution by reducing the complexity of each individual step.
• Reusability: Intermediate tables or results can be reused across multiple queries within the same session.

These examples demonstrate how intermediate tables (CTEs, temporary tables, or subqueries) can be used effectively to answer specific queries or perform complex data manipulations in SQL. Each approach has its strengths depending on the specific requirements and constraints of your database and application.