JOIN with CTE (Common Table Expression)

11 Oct 2023

In SQL, the JOIN operation is used to combine rows from two or more tables based on a related column between them. It allows us to retrieve data from multiple tables in a single query. Common Table Expression (CTE) is a temporary named result set which we can reference within a SELECT, INSERT, UPDATE, or DELETE statement. It allows us to create more complex and readable queries.

In this blog post, we will explore how to use a JOIN with CTE in SQL to perform powerful and efficient queries.

Syntax

The syntax for using JOIN with CTE is as follows:

WITH cte_name AS (
    SELECT column1, column2, ...
    FROM table_name
    WHERE condition
)
SELECT columns
FROM table
JOIN cte_name ON table.column = cte_name.column;

Example

Let’s suppose we have two tables, customers and orders, where customers can place multiple orders. We want to retrieve all the customer names along with their respective order count.

WITH order_count AS (
    SELECT customer_id, COUNT(*) AS count
    FROM orders
    GROUP BY customer_id
)
SELECT c.customer_name, o.count
FROM customers c
JOIN order_count o ON c.customer_id = o.customer_id;

Explanation

First, we define a CTE named order_count which selects the customer_id and counts the number of orders for each customer from the orders table.
Next, we join the customers table with the order_count CTE using the customer_id column as the join condition.
Finally, we select the customer_name from the customers table and the count from the order_count CTE.

This will give us the desired result, where each row represents a customer’s name along with their respective order count.

Benefits of JOIN with CTE

Using JOIN with Common Table Expression offers several benefits:

Code reusability: CTE allows us to define a result set that can be referenced multiple times within a query, reducing code duplication.
Readability: CTE enhances query readability by breaking down complex logic into separate, more manageable parts.
Performance optimization: CTE can help optimize query execution plans and improve overall performance by providing intermediate result sets.

Conclusion

JOIN with CTE is a powerful feature in SQL that allows us to combine the simplicity and readability of Common Table Expressions with the flexibility and efficiency of JOIN operations. It enables us to write more complex, yet maintainable queries for retrieving data from multiple tables.

By mastering JOIN with CTE, you can enhance your SQL skills and create more efficient and concise database queries.

#SQL #CTE