Common Table Expression (CTE)

Standard Table Expression SELECT, INSERT, UPDATE, and DELETE statements can all reference a named temporary result set (CTE) within a single SQL statement. CTEs are created by utilizing the SELECT statement, the WITH keyword, the CTE name, and the result set definition.

Here is the syntax of a CTE in SQL as below:

WITH CTE_name AS ( SELECT column_name1, column_name2, ... FROM table_name WHERE condition ) SELECT * FROM CTE_name WHERE condition;

This example uses a CTE called "CTE_name" that, in response to a certain condition, extracts particular columns from a table. The SELECT statement that follows the CTE definition can then make use of the resultant set as a reference.

Here is an example of how a CTE can simplify a complex query:

WITH monthly_sales AS ( SELECT SUM(sales_amount) AS total_sales, MONTH(sales_date) AS month FROM sales GROUP BY MONTH(sales_date) ) SELECT * FROM monthly_sales WHERE total_sales > (SELECT AVG(total_sales) FROM monthly_sales);

This example uses a CTE called "monthly_sales" to determine the monthly total of sales. The resulting set is then put to use in the subsequent SELECT statement to find all months when the total sales exceeded the average monthly sales. The following are CTEs' advantages over standard SQL subqueries:

1. Code Readability: When working with nested subqueries, CTEs can make complex queries simpler to read and comprehend. A result set can be given a temporary name to make it more obvious what it does within the bigger query.

2. Code Reusability: CTEs can be utilized in many inquiries without the need to rewrite the subquery because they can be referenced several times within a single SQL statement.

3. Query optimization: The SQL server can sometimes improve query performance by optimizing CTEs similarly to standard SELECT statements.