<|im_start|>system
You are a helpful AI assistant.<|im_end|>
<|im_start|>user
What is the purpose of common table expressions in SQL and when should you use them?<|im_end|>
<|im_start|>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.<|im_end|>
<|im_start|>user
What are some basic examples?<|im_end|>
<|im_start|>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.<|im_end|>
<|im_start|>user
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.<|im_end|>
<|im_start|>assistant
Sure, here is an example of how you could use a Common Table Expression (CTE) to store information about game design for a survival game [1][2]. Let's say you have a crafting system in your game where players can create different items. You can use a CTE to store information about the required components for each item, which could include the item's name, the required components, and the amounts of each component needed.

For example, let's say you have an item called "Armor" which requires 5 wood, 2 iron, and 1 leather. You can define this CTE as follows:

WITH CraftingComponents AS ( SELECT 'Armor' AS ItemName, 'Wood' AS Component, 5 AS Amount UNION SELECT 'Armor', 'Iron', 2 UNION SELECT 'Armor', 'Leather', 1 )

You can then use the CTE to access this information in your code. In Unity, you can use the C# language to retrieve the components for an item in the game. Here is example code that does this:

// Retrieve components for an item string itemName = "Armor";

// Get components from CTE var query = from c in CraftingComponents where c.ItemName == itemName select c;

// Iterate through components foreach(var component in query) { string componentName = component.Component; int amountNeeded = component.Amount;

// Do something with the component }

With the CTE, you can easily access the components and amounts needed for any item in the game. This makes it easier to manage the components and update them if needed.<|im_end|>