Understanding NULLIF and ISNULL
Introduction
One of my friends wants to know about two functions NULLIF and ISNULL. In this article I am trying to discuss about that.
Understanding NULLIF
First we look at the syntax of the NULLIF
The syntax is:
NULLIF(Expression, Expression)
Return type:
Return type is the same type of the first expression or NULL
How it works:
NULLIF is equivalent to a searched CASE expression in which the two expressions are equal and the resulting expression is NULL. NULLIF returns the first expression if the two expressions are not equal.
Example of NULLIF
Step-1 [ Create the Base Table ]
IF OBJECT_ID('Tbl_DEVGROUP') IS NOT NULL
BEGIN
DROP TABLE Tbl_DEVGROUP;
END
GO
CREATE TABLE Tbl_DEVGROUP
(DEVID INT NOT NULL IDENTITY PRIMARY KEY,
DEVNAME VARCHAR(50) NOT NULL,
LANG VARCHAR(50) NOT NULL);
GO
INSERT INTO Tbl_DEVGROUP
(DEVNAME, LANG)
VALUES ('SANGRAM', '.NET'),
('TUHIN', 'C++'),
('ROOHUL', 'VC++'),
('SUKAMAL', 'SQL')
GO
Step-2 [ Using NULLIF and Compare the Output ]
-- Returns NULL Value if Expression1 and Expression2 are same
SELECT DEVID, DEVNAME, NULLIF(LANG, '.NET') AS 'Null if Equal'
FROM Tbl_DEVGROUP
WHERE DEVID = 1;
GO
-- Output
DEVID DEVNAME Null if Equal
1 SANGRAM NULL
GO
-- Returns Expression1 Value if Expression1 and Expression2 are not same
SELECT DEVID, DEVNAME, NULLIF(LANG, 'C++') AS 'Null if Equal'
FROM Tbl_DEVGROUP
WHERE DEVID = 1;
GO
-- Output
DEVID DEVNAME Null if Equal
1 SANGRAM .NET
Understanding ISNULL
First of all we look at the syntax.
The syntax is:
ISNULL ( check_expression , replacement_value )
Arguments:
check_expression
Is the expression to be checked for NULL. check_expression can be of any type.
replacement_value
Is the expression to be returned if check_expression is NULL. replacement_value must be of a type that is implicitly convertible to the type of check_expresssion.
Return Type:
Return the same type of check expression.
How it works:
If the Check expression is not null then it returns the check expression. If the check expression is NULL it returns the replacement value.
Example of ISNULL
Step-1 [ Create the Base Table ]
IF OBJECT_ID('Tbl_DEVGROUP') IS NOT NULL
BEGIN
DROP TABLE Tbl_DEVGROUP;
END
GO
CREATE TABLE Tbl_DEVGROUP
(DEVID INT NOT NULL IDENTITY PRIMARY KEY,
DEVNAME VARCHAR(50) NOT NULL,
LANG VARCHAR(50) NULL);
GO
INSERT INTO Tbl_DEVGROUP
(DEVNAME, LANG)
VALUES ('SANGRAM', '.NET'),
('TUHIN', NULL),
('ROOHUL', 'VC++'),
('SUKAMAL', NULL)
GO
Step-2 [ Using ISNULL ]
-- Checking the Null Value only
SELECT DEVID, DEVNAME, LANG
FROM Tbl_DEVGROUP
WHERE LANG IS NULL;
-- Alternate way
SELECT DEVID, DEVNAME, LANG
FROM Tbl_DEVGROUP
WHERE ISNULL(LANG, '') = '';
GO
-- Output
DEVID DEVNAME LANG
2 TUHIN NULL
4 SUKAMAL NULL
-- Null Replacement with ISNULL
SELECT DEVID, DEVNAME, ISNULL(LANG, 'NOT DEFINE') AS LANG
FROM Tbl_DEVGROUP;
GO
-- Output
DEVID DEVNAME LANG
1 SANGRAM .NET
2 TUHIN NOT DEFINE
3 ROOHUL VC++
4 SUKAMAL NOT DEFINE
Hope you like it.
SELECT ISNULL(ISNULL(Col1, Col2), Col3)
What to use ISNULL() or COALESCE()
Introduction
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 athttp://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.
Let's start
The NULL value always makes some problem to database developer. To handle the null value properly the SQL server provides two functions:
1. ISNULL()
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
Roll
|
StudentName
|
StudentClass
|
1
|
JOYDEEP
|
1
|
2
|
TUHIN
|
1
|
3
|
NULL
|
1
|
To handle the NULL value with ISNULL()
SELECT ISNULL(StudentName,'NOT FOUND') StudentName
FROM Table1-A
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
);
GO
INSERT dbo.wages
(hourly_wage, salary, commission, num_sales)
VALUES
(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);
GO
SELECT
CAST(COALESCE(hourly_wage * 40 * 52, salary,
commission * num_sales) AS money) AS 'Total Salary'
FROM dbo.wages
ORDER BY 'Total Salary';
The Output is mentioned bellow
Total Salary
10000.00
20000.00
20800.00
30000.00
40000.00
41600.00
45000.00
50000.00
56000.00
62400.00
83200.00
120000.00
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),
@Field2 char(50)
SET @Field2 = 'Some Long String'
SELECT ISNULL(@Field1, @Field2)
--Returns 'Som'
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().
