A Common Table Expression (CTE) is a temporary result set that you can reference within a SQL query. CTEs are particularly useful for breaking down complex queries into more manageable and readable parts. They also provide a way to reuse intermediate results and avoid redundant code, which can improve the overall efficiency of your queries.

CTEs are defined using the WITH clause and are typically followed by a SELECT statement that uses the defined CTE. The CTE is only available within the query that follows the WITH clause. It’s a great way to enhance the readability and maintainability of your SQL code.

Here’s the basic syntax of a CTE:

WITH cte_name (column1, column2, …) AS (
-- CTE definition
SELECT …
)
-- Main query that uses the CTE
SELECT …

Let’s look at an example to better understand how CTEs work:

Suppose you have a database with a table named Employees, and you want to find the average salary of employees in each department. Additionally, you want to exclude departments where the average salary is below a certain threshold.

Here’s how you could achieve this using a CTE:

WITH DepartmentSalaries AS (
SELECT
DepartmentID,
AVG(Salary) AS AvgSalary
FROM Employees
GROUP BY DepartmentID
)
SELECT
DepartmentID,
AvgSalary
FROM DepartmentSalaries
WHERE AvgSalary >= 50000; -- Only show departments with average salary >= 50000

A Common Table Expression (CTE) is a temporary result set that you can reference within a SQL query. CTEs are particularly useful for breaking down complex queries into more manageable and readable parts. They also provide a way to reuse intermediate results and avoid redundant code, which can improve the overall efficiency of your queries.

Example COMMON TABLE EXPRESSION from todo project:-

WITH
created_cte AS(
SELECT NAME
,
AVG(users.id) AS Average_users
FROM
users
GROUP BY NAME
)
SELECT NAME
,
Average_users
FROM
created_cte
WHERE
Average_users >= 15;

CTEs are defined using the WITH clause and are typically followed by a SELECT statement that uses the defined CTE. The CTE is only available within the query that follows the WITH clause. It’s a great way to enhance the readability and maintainability of your SQL code.

Here’s the basic syntax of a CTE:

sqlCopy codeWITH cte_name (column1, column2, ...) AS (
    -- CTE definition
    SELECT ...
)
-- Main query that uses the CTE
SELECT ...

Let’s look at an example to better understand how CTEs work:

Suppose you have a database with a table named Employees, and you want to find the average salary of employees in each department. Additionally, you want to exclude departments where the average salary is below a certain threshold.

Here’s how you could achieve this using a CTE:

sqlCopy codeWITH DepartmentSalaries AS (
    SELECT
        DepartmentID,
        AVG(Salary) AS AvgSalary
    FROM Employees
    GROUP BY DepartmentID
)
SELECT
    DepartmentID,
    AvgSalary
FROM DepartmentSalaries
WHERE AvgSalary >= 50000; -- Only show departments with average salary >= 50000

In this example, the CTE named DepartmentSalaries calculates the average salary for each department by using a GROUP BY clause on the DepartmentID. The main query then selects the DepartmentID and AvgSalary from the CTE and applies a filter to show only those departments where the average salary is greater than or equal to 50000.

CTEs are especially valuable when you have more complex scenarios, such as recursive queries or multiple levels of subqueries, where breaking down the logic into smaller, manageable parts can greatly enhance the readability and maintainability of your SQL code.

Benefits of using CTEs:

1. Improved readability: CTEs allow you to define named result sets for specific parts of your query, making it easier to understand the logic.
2. Code reusability: You can reference the CTE multiple times within the same query, reducing redundant code.
3. Simplified debugging: By isolating complex subqueries or intermediate results, you can focus on debugging smaller sections of your query.
4. Optimizer benefits: The database optimizer may be able to optimize CTEs better than subqueries, leading to potentially improved performance.

Remember that not all database systems support CTEs, so it’s essential to check the documentation of your specific database to ensure compatibility.