Returns a null value if the two specified expressions are equal.
NULLIF is equivalent to a searched CASE expression in which the two expressions are equal and the resulting expression is NULL.
We recommend that you not use time-dependent functions, such as RAND(), within a NULLIF function. This could cause the function to be evaluted twice and to return different results from the two invocations.
A. Returning budget amounts that have not changed
The following example creates a budgets table to show a department (dept) its current budget (current_year) and its previous budget (previous_year). For the current year, NULL is used for departments with budgets that have not changed from the previous year, and 0 is used for budgets that have not yet been determined. To find out the average of only those departments that receive a budget and to include the budget value from the previous year (use the previous_year value, where the current_year is NULL), combine the NULLIF and COALESCE functions.
USE AdventureWorks2008R2; GO IF OBJECT_ID ('dbo.budgets','U') IS NOT NULL DROP TABLE budgets; GO SET NOCOUNT ON; CREATE TABLE dbo.budgets ( dept tinyint IDENTITY, current_year decimal NULL, previous_year decimal NULL ); INSERT budgets VALUES(100000, 150000); INSERT budgets VALUES(NULL, 300000); INSERT budgets VALUES(0, 100000); INSERT budgets VALUES(NULL, 150000); INSERT budgets VALUES(300000, 250000); GO SET NOCOUNT OFF; SELECT AVG(NULLIF(COALESCE(current_year, previous_year), 0.00)) AS 'Average Budget' FROM budgets; GO
Here is the result set.
(1 row(s) affected)
B. Comparing NULLIF and CASE
To show the similarity between NULLIF and CASE, the following queries evaluate whether the values in the MakeFlag and FinishedGoodsFlag columns are the same. The first query uses NULLIF. The second query uses the CASE expression.
USE AdventureWorks2008R2; GO SELECT ProductID, MakeFlag, FinishedGoodsFlag, NULLIF(MakeFlag,FinishedGoodsFlag)AS 'Null if Equal' FROM Production.Product WHERE ProductID < 10; GO SELECT ProductID, MakeFlag, FinishedGoodsFlag,'Null if Equal' = CASE WHEN MakeFlag = FinishedGoodsFlag THEN NULL ELSE MakeFlag END FROM Production.Product WHERE ProductID < 10; GO