NVL Function in Oracle

The NVL function in Oracle is used to replace NULL values with a specified default value. If the value in the first parameter is NULL, NVL returns the second parameter. If the value in the first parameter is not NULL, NVL returns the first parameter.

Here's the syntax of the NVL function:

NVL(expression, default_value)

- expression: The value to check for NULL.

- default_value: The value to return if expression is NULL.

Example:

SELECT NVL(salary, 0) AS salary_with_default

FROM employees;

In this example, if the salary column contains NULL values, the NVL function will return 0 for those records. Otherwise, it will return the value of the salary column.

Here's another example using the NVL function in Oracle:

Suppose we have a table called employees with columns employee_id, first_name, last_name, and hire_date. Some of the hire_date values are NULL. We want to retrieve the hire_date values with a default value of 'Not Available' for the NULL entries.

Here's how we can use the NVL function to achieve this:

SELECT employee_id, first_name, last_name, NVL(hire_date, 'Not Available') AS hire_date

FROM employees;

In this example:

- We select the employee_id, first_name, last_name, and hire_date columns from the employees table.

- We use the NVL function to replace NULL values in the hire_date column with the default value Not Available.

- The query will return the employee_id, first_name, last_name, and hire_date columns, with NULL values in the hire_date column replaced by Not Available. 

Here are 5 frequently asked questions (FAQs) about the NVL function in Oracle:

1. What is the NVL function used for in Oracle?

   - The NVL function is used to replace NULL values with a specified default value in Oracle SQL queries.

2. How does the NVL function work?

   - The NVL function takes two parameters: the expression to check for NULL, and the default value to return if the expression is NULL. If the expression is not NULL, NVL returns the expression; otherwise, it returns the default value.

3. Can I use NVL with any data type in Oracle?

   - Yes, the NVL function can be used with any data type in Oracle, including VARCHAR2, NUMBER, DATE, and others.

4. Is NVL the same as COALESCE in Oracle?

   - While both NVL and COALESCE are used to handle NULL values, there are some differences. NVL takes only two parameters, while COALESCE can take multiple parameters and returns the first non-NULL value. Additionally, NVL is Oracle-specific, while COALESCE is a standard SQL function supported by multiple database systems.

5. Can I nest NVL functions in Oracle?

   - Yes, you can nest NVL functions in Oracle to handle multiple levels of NULL replacement. However, it's important to keep the performance implications in mind when nesting functions excessively.