One of my Blog post is very much appritiated by my reader is different between ISNULL() and COALESCE (). But in early time I post my blogs at http://sqlservernet.blogspot.in/2012/03/what-to-use-isnull-or-coalesce.html
So I decide to move this post in my web that all my reader can enjoy it.
The NULL value always makes some problem to database developer. To handle the null value properly the SQL server provides two functions:
2. COALESCE ()
Both functions act like same.
Before finding the difference between two functions we must understand, what is the functionality of both functions to take care of null values.
Suppose we have a Table names TBL-A
To handle the NULL value with ISNULL()
SELECT ISNULL(StudentName,'NOT FOUND') StudentName
It returns "NOT FOUND" in studentname columns for roll = 3
The syntax of the COALESCE is mentioned bellow
COALESCE ( expression [ ,...n ] )
Unlike ISNULL, COALESCE takes multiple columns as expression. To understand it properly I give an example. Please study the example well and you will understand the COALESCE
CREATE TABLE dbo.wages
emp_id tinyint identity,
hourly_wage decimal NULL,
salary decimal NULL,
commission decimal NULL,
num_sales tinyint NULL
(hourly_wage, salary, commission, num_sales)
(10.00, NULL, NULL, NULL),
(20.00, NULL, NULL, NULL),
(30.00, NULL, NULL, NULL),
(40.00, NULL, NULL, NULL),
(NULL, 10000.00, NULL, NULL),
(NULL, 20000.00, NULL, NULL),
(NULL, 30000.00, NULL, NULL),
(NULL, 40000.00, NULL, NULL),
(NULL, NULL, 15000, 3),
(NULL, NULL, 25000, 2),
(NULL, NULL, 20000, 6),
(NULL, NULL, 14000, 4);
CAST(COALESCE(hourly_wage * 40 * 52, salary,
commission * num_sales) AS money) AS 'Total Salary'
ORDER BY 'Total Salary';
The Output is mentioned bellow
Now what are the differences between them?
1. COALESCE is ANSI-92 compliant and ISNULL is not. So if u migrate the SQL statement in any other RDBMS the COALESCE is helpful.
2. COALESCE accepts greater than two expressions, whereas ISNULL accepts only two. In order to compare three expressions with ISNULL, you would have to nest expressions:
SELECT ISNULL(ISNULL(Col1, Col2), Col3)
3. In this SQL the ISNULL can provide some Error
DECLARE @Field1 char(3),
SET @Field2 = 'Some Long String'
SELECT ISNULL(@Field1, @Field2)
SELECT COALESCE(@Field1, @Field2)
--Returns 'Some Long String'
4. Performance wise ISNULL and COALESCE are equal. It is found that the ISNULL is
slightly better performer then COALESCE.
In my advice use COALESCES() rather than ISNULL().
Hope you like this.
Posted by: MR. JOYDEEP DAS