Temp_tables_ctes.md

Temporary Tables and Common Table Expressions (CTEs)

Introduction

Temporary tables and Common Table Expressions (CTEs) are useful for breaking down complex queries and storing intermediate results. This section will cover how to create and use temporary tables and CTEs in SQL, including MySQL, PostgreSQL, SQL Server, and DuckDB.

Temporary Tables

What are Temporary Tables?

Temporary tables are tables that exist temporarily during the session or until the query that created them ends. They are useful for storing intermediate results that need further processing.

Creating Temporary Tables

The syntax for creating a temporary table varies slightly between SQL dialects.

MySQL

CREATE TEMPORARY TABLE temp_table_name (
    column1 datatype,
    column2 datatype,
    ...
);

PostgreSQL

CREATE TEMP TABLE temp_table_name (
    column1 datatype,
    column2 datatype,
    ...
);

SQL Server

CREATE TABLE #temp_table_name (
    column1 datatype,
    column2 datatype,
    ...
);

DuckDB

CREATE TEMP TABLE temp_table_name (
    column1 datatype,
    column2 datatype,
    ...
);

Example

Create a temporary table to store intermediate results:

CREATE TEMP TABLE temp_sales (
    product_id INT,
    total_sales DECIMAL(10, 2)
);

INSERT INTO temp_sales (product_id, total_sales)
SELECT
  productid,
  SUM(amount)
FROM sales
GROUP BY product_id;

SELECT * FROM temp_sales;

Dropping Temporary Tables

Temporary tables are automatically dropped at the end of the session. However, you can explicitly drop them.

DROP TABLE temp_sales;

Common Table Expressions (CTEs)

What are CTEs?

CTEs are temporary result sets that are defined within the execution scope of a single SELECT, INSERT, UPDATE, or DELETE statement. They make queries easier to read and maintain by breaking down complex queries.

Creating CTEs

CTEs are created using the WITH keyword followed by the CTE name and the query that defines it.

WITH cte_name AS (
    SELECT column1, column2, ...
    FROM table_name
    WHERE condition
)
SELECT column1, column2, ...
FROM cte_name;

Example Using a CTE to calculate total sales and then retrieve products with sales greater than 1000:

WITH TotalSales AS (
    SELECT productid, SUM(amount) AS total_sales
    FROM sales
    GROUP BY productid
)
SELECT productid, total_sales
FROM TotalSales
WHERE total_sales > 1000;

Recursive CTEs

Recursive CTEs are useful for hierarchical or recursive data structures such as organizational charts or tree structures.

WITH RECURSIVE cte_name AS (
    -- Anchor member
    SELECT column1, column2, ...
    FROM table_name
    WHERE condition

    UNION ALL

    -- Recursive member
    SELECT column1, column2, ...
    FROM table_name
    INNER JOIN cte_name ON table_name.column = cte_name.column
)
SELECT column1, column2, ...
FROM cte_name;

Example

Using a recursive CTE to retrieve an employee hierarchy:

WITH RECURSIVE EmployeeHierarchy AS (
    SELECT employeeid, managerid, firstname, lastname
    FROM employees
    WHERE manager_id IS NULL

    UNION ALL

    SELECT e.employeeid, e.managerid, e.firstname, e.lastname
    FROM employees e
    INNER JOIN EmployeeHierarchy eh ON e.manager_id = eh.employee_id
)
SELECT employeeid, firstname, lastname, managerid
FROM EmployeeHierarchy;

Practice Exercises

• Create a CTE containting all employee that work in marketing, use this in your select statement to get the firstname and lastname of these employees.
• create a temp table with the sum of all sales, how query this table working out how much this would be if 50% of the total sales was added oon.

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