Common Table Expressions (CTEs) for Simplifying SQL Queries

Introduction

As developers, we often encounter complex SQL queries that can be challenging to write and maintain. One technique that can help simplify these queries is the use of Common Table Expressions (CTEs). In this article, we will explore CTEs in detail, including how they work, when to use them, and provide examples to illustrate their application.

What are Common Table Expressions?

Common Table Expressions (CTEs) are temporary result sets that can be referenced within a SQL query. They allow us to simplify complex queries by breaking them down into smaller, more manageable pieces. CTEs can be used in various contexts, such as aggregations, joins, and subqueries.

How Do CTEs Work?

A CTE is defined using the WITH keyword followed by the name of the CTE and an optional description. The CTE is then referenced within the main query using its name. Here’s a simplified example:

with my_cte as (
    select * from my_table
)
select * from my_cte;

In this example, my_cte is a CTE that references the entire table my_table. The result of the subquery within the CTE is then returned by the main query.

Types of CTEs

There are two types of CTEs:

• Temporary CTE: A temporary CTE is defined using the WITH keyword and is valid only for the duration of the current SQL statement.
• Persistent CTE: A persistent CTE is defined using the WITH RECURSIVE keyword and can be referenced multiple times within a query.

When to Use CTEs?

CTEs are useful in various scenarios:

• Aggregation: CTEs can simplify complex aggregations by breaking them down into smaller pieces.
• Joining: CTEs can help join multiple tables together, making the query more readable and maintainable.
• Subqueries: CTEs can replace subqueries with a more efficient and scalable solution.

Example: Using CTEs to Simplify a Query

Let’s revisit the original question about calculating the global price for each id_test object. The goal is to sum up the product of quantity and price along with the fee, while ensuring that the fee is added only once per id_test.

-- Original query without CTEs
SELECT 
    SUM(quantity * price) AS quantity,
    SUM(fee) AS fee
FROM data;

-- Query using CTEs
with total_fee as (
    select distinct id_test, fee from data
),
total_price as (
    select distinct id_test, sum(quantity) quantity, sum(price) price 
    from data
    group by id_test
)
SELECT 
    SUM(b.price * b.quantity) AS quantity,
    SUM(a.fee) AS fee
FROM total_fee a
LEFT JOIN total_price b ON a.id_test = b.id_test;

In this example, the CTEs simplify the original query by breaking it down into smaller pieces. The total_fee CTE extracts unique id_test and fee combinations, while the total_price CTE calculates the sum of quantity and price for each id_test. The main query then joins these two result sets together to produce the final output.

Benefits of Using CTEs

Using CTEs offers several benefits:

• Improved readability: CTEs can simplify complex queries by breaking them down into smaller, more manageable pieces.
• Reduced maintenance: CTEs make it easier to modify or update a query without affecting other parts of the application.
• Increased scalability: CTEs can be used in conjunction with other optimization techniques to improve query performance.

Conclusion

Common Table Expressions (CTEs) are a powerful tool for simplifying SQL queries. By breaking down complex queries into smaller pieces, CTEs can improve readability, reduce maintenance, and increase scalability. In this article, we explored the basics of CTEs, including how they work, when to use them, and provided examples to illustrate their application. We also discussed the benefits of using CTEs and demonstrated their value in simplifying a complex query.

Additional Resources

For more information on CTEs, refer to the following resources:

• [SQL Fiddle: Common Table Expressions](https://sqlfiddle.com/ /queries/2e5c9a)
• W3Schools: Common Table Expressions
• Database Tutorial: Common Table Expressions

Last modified on 2023-11-27