What is CTE in SQL

 

Common table Expression

CTE in SQL

Common Table Expressions (CTE) is a feature in SQL that allows you to define a temporary result set that can be referenced within a SQL statement. CTEs provide a way to simplify complex queries and make them more readable.

History of CTE:

The concept of CTEs was introduced in SQL:1999 standard. Initially, it was supported only by a few databases, but now it is a widely used feature in many SQL-based databases.

Types of CTE:

There are two types of CTEs:

1. Recursive CTE:
• A recursive CTE is used when we need to traverse a hierarchical structure, such as an organization chart or a bill of materials. It uses a union of a base query and a recursive query to traverse the structure until it reaches the end.
2. Non-Recursive CTE:
• A non-recursive CTE is used when we need to simplify complex queries by breaking them into smaller, more manageable parts. It uses a simple SELECT statement to define the temporary result set.

Syntax:

The syntax for creating a CTE is as follows:

WITH cte_name (column1, column2, ...) AS ( -- subquery that defines the CTE )

• The "WITH" keyword is used to specify the CTE. The "cte_name" is the name of the CTE
• The "column1", "column2", ... are the columns returned by the CTE.
• The subquery that defines the CTE is enclosed in parentheses and follows the "AS" keyword.

Use Case:

One of the most common use cases for CTEs is to simplify complex queries by breaking them into smaller parts. For example, consider a database that contains information about a company's sales. We want to find the total sales for each salesperson, but we also want to show the total sales for the entire company.

WITH sales_by_salesperson AS ( SELECT salesperson_id, SUM(sales_amount) AS total_sales FROM sales GROUP BY salesperson_id ), total_sales AS ( SELECT SUM(sales_amount) AS total_sales FROM sales )

SELECT salesperson_id, total_sales, total_sales.total_sales AS company_total_sales FROM sales_by_salesperson CROSS JOIN total_sales;

In this example, we define two CTEs.

• The first CTE, "sales_by_salesperson", calculates the total sales for each salesperson.
• The second CTE, "total_sales", calculates the total sales for the entire company. We then join these two CTEs to produce the final result set.
• The output of this query is a table that shows the total sales for each salesperson, as well as the total sales for the entire company.

In conclusion, CTEs are a powerful feature in SQL that allows you to simplify complex queries and make them more readable. They provide a way to define a temporary result set that can be referenced within a SQL statement. By selecting the appropriate CTE type and using the correct syntax, you can make your SQL queries more efficient and easier to understand.