Friday, November 11, 2022

HOW TO USE WITH IN ORACLE?

 The WITH clause creates a temporary table, or it can be seen as an inline or embedded view. Its purpose is to reduce the number of times a particular query is called in a Select statement. The end result is to improve performance significantly. 

In Oracle PL/SQL, the WITH clause creates a temporary result set called a Common Table Expression (CTE). It's beneficial when you want to break down a complex query into more manageable parts, improve query readability, and avoid writing subqueries multiple times. Here's how to use the WITH clause in Oracle PL/SQL:


WITH cte_name (column1, column2, ...) AS ( 
    SELECT ... 
    FROM ... 
    WHERE ... 
)
SELECT * 
FROM cte_name;

Let's break down this structure:

  • cte_name: This is the name you give to your Common Table Expression.

  • (column1, column2, ...): These are the columns you define for your CTE. They don't need to be the same as the columns in the underlying tables; you can transform data in the CTE.


  • SELECT ... FROM ... WHERE ...: This is the query that defines your CTE. You can use any valid SQL statements here.


  • SELECT * FROM cte_name;: Finally, you use the CTE in the main query, treating it like a table.

Here's an example:

Suppose you have a table named employees with columns employee_id, first_name, last_name, and salary. You want to retrieve employees with a salary greater than the average salary.


WITH avg_salary AS ( 
    SELECT AVG(salary) AS average_salary 
    FROM employees 
) 
SELECT employee_id, first_name, last_name, salary 
FROM employees 
WHERE salary > (SELECT average_salary FROM avg_salary);

In this example, the CTE named avg_salary calculates the average salary from the employees table. The main query then selects employees with salaries greater than the calculated average salary from the CTE.

Remember that CTEs are only available within the context of the query where they are defined. They can't be referenced outside of that query. This makes them a great tool for simplifying complex queries while maintaining readability.


Hope you have a nice day!!!




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