prompter: What is the purpose of common table expressions in SQL and when should you use them?

assistant: Common table expressions are temporary result sets that can be utilized in SELECT, INSERT, UPDATE, or DELETE statements. They simplify complex queries and improve readability by breaking down the query into smaller, more manageable pieces. They also have the potential to improve performance in large, complex queries and are particularly useful when working with hierarchical or recursive data. In summary, common table expressions are a valuable tool for optimizing SQL queries and making them more readable and maintainable.

prompter: Could you include small examples?

assistant: A Common Table Expression (CTE) is a temporary result set derived from a SELECT statement, and can be used in a SELECT, INSERT, UPDATE, or DELETE statement. CTEs are used to simplify complex queries by breaking them down into smaller, more manageable pieces.

The purpose of CTEs is to make queries easier to read, write and maintain. CTEs can be thought of as named subqueries or temporary views that are only visible within the scope of a single SELECT, INSERT, UPDATE, or DELETE statement. They are not stored as objects in the database and do not impact database performance.

CTEs are particularly useful when dealing with complex queries that involve multiple levels of data aggregation or calculations, or when the same subquery is used in multiple places in a larger query. Using a CTE allows you to write the subquery once and reuse it throughout the query, making the overall structure of the query simpler and easier to understand.

In terms of when to use CTEs, they are a good choice when you want to:

Break down a complex query into smaller, more manageable pieces.
Improve query readability and maintainability by giving meaningful names to intermediate results.
Reuse intermediate results within the same query.
Simplify complex calculations or aggregations by breaking them down into multiple steps.
In general, if you have a query that is becoming difficult to understand or maintain due to its complexity, a CTE may be a good solution to simplify the query and make it easier to work with.

prompter: What are some basic examples?

assistant: Let’s say you have a table called schools with the columns ```school_id```, ```school_name```, ```district_id```, and the number of students.
You need to write a query to display a list of schools alongside their district ID and the average number of students per school in that district.

Your logic might be as follows:

    Create a table with the list of districts and the corresponding average number of students per school.
    Join this table with the list of schools and display the required information.
    Drop the table with the average number of students per school for each district.

If you use a CTE (Common Table Expression), you don’t need to create and drop a table. You can simply reference the temporary result set created by the "WITH" query, like you see below:

```
WITH avg_students AS (
SELECT district_id, AVG(students) as average_students
FROM schools
GROUP BY district_id)
SELECT s.school_name, s.district_id, avg.average_students
FROM schools s
JOIN avg_students avg
ON s.district_id = avg.district_id;
```

So, here you start by defining the temporary result set ```avg_students``` in the "WITH" query. In the brackets, there is a "SELECT" statement that defines this result set; it contains a list of districts and the corresponding average number of students per school. After the closing parenthesis, you start the main "SELECT" statement. Notice that you reference the temporary result set just like an ordinary table, using its assigned name (```avg_students```). The output will include the school name, the ID of its district, and the average number of students in this district. 


| school_name | district_id | average_students |
| --- | --- | --- |
| Happy Kid | 2 | 238 |
| Smart | 2 | 238 |
| Sun | 5 | 176 |
| Montessori | 5 | 176 |


In the sense that it can’t be used in any other SQL queries, the temporary result set is “dropped”. Of course, you can define it again if you need to.

assistant: Here are some basic examples of using CTEs in SQL:

Calculating the cumulative sum of a column:

WITH cumulative_sum AS (
  SELECT 
    order_id, 
    order_date, 
    order_total, 
    SUM(order_total) OVER (ORDER BY order_date) AS running_total
  FROM orders
)
SELECT 
  order_id, 
  order_date, 
  order_total, 
  running_total
FROM cumulative_sum;

Breaking down a complex query into multiple steps:

WITH customer_info AS (
  SELECT 
    customer_id, 
    customer_name, 
    city, 
    state
  FROM customers
),
order_info AS (
  SELECT 
    order_id, 
    customer_id, 
    order_date, 
    order_total
  FROM orders
)
SELECT 
  customer_info.customer_name, 
  order_info.order_date, 
  order_info.order_total
FROM customer_info
JOIN order_info 
  ON customer_info.customer_id = order_info.customer_id;

Reusing intermediate results:

WITH top_customers AS (
  SELECT 
    customer_id, 
    SUM(order_total) AS total_spend
  FROM orders
  GROUP BY customer_id
),
top_10_customers AS (
  SELECT 
    customer_id
  FROM top_customers
  ORDER BY total_spend DESC
  LIMIT 10
)
SELECT 
  customer_id, 
  order_date, 
  order_total
FROM orders
WHERE customer_id IN (SELECT customer_id FROM top_10_customers);

In these examples, the CTEs are used to simplify the overall structure of the query and make it easier to understand and maintain.

assistant: Common Table Expressions (CTEs) are a feature in SQL that allow you to define a temporary named result set that can be used within a SELECT, INSERT, UPDATE, or DELETE statement. They provide a way to simplify complex queries and improve readability by breaking down a query into smaller, reusable parts.

CTEs can be thought of as named subqueries, as they can be used to encapsulate a query and refer to it later in the same statement. The key difference between subqueries and CTEs is that subqueries are unnamed and can only be used within the same query, while CTEs can be named and can be referred to multiple times within a query.

You should use CTEs when you have a complex query that needs to be broken down into smaller, more manageable parts. They can also be used to improve the readability and maintainability of a query by giving meaningful names to intermediate results. Additionally, CTEs can provide performance benefits, as the query optimizer may be able to use the results of a CTE multiple times without having to recompute them each time.

Here's an example of a query that uses a CTE:

vbnet

WITH TotalSales AS (
   SELECT CustomerID, SUM(Amount) AS TotalAmount
   FROM Orders
   GROUP BY CustomerID
)
SELECT Customers.Name, TotalAmount
FROM Customers
JOIN TotalSales ON Customers.CustomerID = TotalSales.CustomerID

In this example, the CTE named "TotalSales" is used to compute the total sales for each customer. The results of the CTE are then used in the main query to join with the customers table and retrieve the customer name and total sales.

prompter: Can you please give me another example?   Specifically, use an example of when I might use them to store information about game design, perhaps with data surrounding the concept of inventory and crafting for a survival game.   I need to use this in unity.