Oracle Function： NVL

Description

The Oracle/PLSQL NVL function lets you substitute a value when a null value is encountered.

NVL函数是当出现空值时替换一个值

Syntax

NVL( string1, replace_with )

String1	The string to test for a null value.
replace_with	The value returned if string1 is null.

Example

For example:

SELECT NVL(supplier_city, 'n/a') FROM suppliers;	The SQL statement above would return 'n/a' if the supplier_city field contained a null value. Otherwise, it would return the supplier_city value.
SELECT supplier_id, NVL(supplier_desc, supplier_name) FROM suppliers;	This SQL statement would return the supplier_name field if the supplier_desc contained a null value. Otherwise, it would return the supplier_desc.
SELECT NVL(commission, 0) FROM sales;	This SQL statement would return 0 if the commission field contained a null value. Otherwise, it would return the commission field.

Frequently Asked Questions

Question:	Answer:
I tried to use the NVL function through VB to access Oracle DB. To be precise, SELECT NVL(Distinct (emp_name),'AAA'),................ FROM................. I got an oracle error when I use distinct clause with NVL, but when I remove distinct it works fine.	It is possible to the use the DISTINCT clause with the NVL function. However, the DISTINCT must come before the use of the NVL function. For example: SELECT distinct NVL(emp_name, 'AAA') FROM employees;
Is it possible to use the NVL function with more than one column with the same function call? To be clear, if i need to apply this NVL function to more than one column like this: NVL(column1;column2 ...... , here is the default value for all )	Answer: You will need to make separate NVL function calls for each column. For example: SELECT NVL(table_name, 'not found'), NVL(owner, 'not found') FROM all_tables;
Examine the TRAINING table as given below: Name                                      Null?    Type  ----------------------------------------- -------- ------------- TRAINING_ID                               NOT NULL NUMBER(5)  TRAINING_LOCATION                                  NUMBER(7,2)  START_DATE                                         DATE  END_DATE                                           DATE Which two SQL would execute successfully? (Choose two) SELECT NVL (ADD_MONTHS (END_DATE,1),SYSDATE) FROM training; SELECT TO_DATE (NVL(SYSDATE-END_DATE,SYSDATE)) FROM training; SELECT NVL(MONTHS_BETWEEN(START_DATE,END_DATE),’In Progress’) FROM training; SELECT NVL(TO_CHAR(MONTHS_BETWEEN(START_DATE,END_DATE)),’In Progress’) FROM training;	A, D. Use NVL function to provide an alternate value to a column when NULL.

使用DISTINCT 的方法COUNT函数和NVL函数的区别：

NVL	SELECT DISTINCT NVL(emp_name, 'AAA') FROM employees;
COUNT	SELECT COUNT(DISTINCT department) AS "Unique departments" FROM employees WHERE salary > 55000;

From <https://www.techonthenet.com/oracle/functions/nvl.php>