How to use with in oracle

The WITH statement improves the readability, reusability, and performance of Oracle queries by defining a temporary table expression (CTE): Define the CTE: WITH AS () Using the CTE: SELECT ... FROM ;Benefits include improved readability, avoiding duplication of subqueries, and optimizing performance through precomputation.

With Statement Usage in Oracle

The WITH statement is a syntax structure that can be used to define temporary Table expressions (CTE), which can be reused in queries. It provides the convenience of improving code readability and performance.

Usage:

<code>WITH <CTE_name> AS (
  <subquery>
)
SELECT ...
FROM <CTE_name>;</code>

Benefits:

• Improve readability: WITH statements encapsulate complex subqueries in named CTEs, making the code easier to understand and maintain.
• Reusability: A CTE can be referenced multiple times in a query to avoid writing the same subquery repeatedly.
• Performance Optimization: Oracle optimizer precomputes CTE, reducing access to the underlying table, thereby improving performance.

Example:

<code>WITH EmployeeInfo AS (
  SELECT employee_id, salary, department_id
  FROM employees
)
SELECT e.employee_id, e.salary, d.department_name
FROM EmployeeInfo e
JOIN departments d ON e.department_id = d.department_id;</code>

In this example, the EmployeeInfo CTE selects employee information from the employees table . The main query then retrieves and joins data from the EmployeeInfo CTE and departments tables to get the employee details and department name.

Usage Notes:

• The subquery in a CTE cannot reference the CTE itself.
• CTE names must be unique.
• CTE is only valid within the current query scope.

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