SQL NULL Functions
SQL ISNULL(), NVL(), IFNULL() and COALESCE() Functions
Look at the following "Products" table:
| P_Id |
ProductName |
UnitPrice |
UnitsInStock |
UnitsOnOrder |
| 1 |
Jarlsberg |
10.45 |
16 |
15 |
| 2 |
Mascarpone |
32.56 |
23 |
|
| 3 |
Gorgonzola |
15.67 |
9 |
20 |
Suppose that the "UnitsOnOrder" column is optional, and may contain NULL values.
We have the following SELECT statement:
SELECT ProductName,UnitPrice*(UnitsInStock+UnitsOnOrder)
FROM Products
|
In the example above, if any of the "UnitsOnOrder" values are NULL, the result is NULL.
Microsoft's ISNULL() function is used to specify how we want to treat NULL values.
The NVL(), IFNULL(), and COALESCE() functions can also be used to achieve the
same result.
In this case we want NULL values to be zero.
Below, if "UnitsOnOrder" is NULL it will not harm
the calculation, because ISNULL() returns a zero if the value is NULL:
SQL Server / MS Access
SELECT ProductName,UnitPrice*(UnitsInStock+ISNULL(UnitsOnOrder,0))
FROM Products
|
Oracle
Oracle does not have an ISNULL() function. However, we can use the NVL() function
to achieve the same result:
SELECT ProductName,UnitPrice*(UnitsInStock+NVL(UnitsOnOrder,0))
FROM Products
|
MySQL
MySQL does have an ISNULL() function. However, it works a little bit
different from Microsoft's ISNULL() function.
In MySQL we can use the IFNULL() function, like this:
SELECT ProductName,UnitPrice*(UnitsInStock+IFNULL(UnitsOnOrder,0))
FROM Products
|
or we can use the COALESCE() function, like this:
SELECT ProductName,UnitPrice*(UnitsInStock+COALESCE(UnitsOnOrder,0))
FROM Products
|
Learn how your website performs under various load conditions
 |
|
WAPT
is a load, stress and performance testing tool for websites and web-based applications.
In contrast to "800-pound gorilla" load testing tools, it is designed to minimize the learning
curve and give you an ability to create a heavy load from a regular workstation.
WAPT is able to generate up to 3000 simultaneously acting virtual users using standard hardware configuration.
Virtual users in each profile are fully customizable. Basic and NTLM authentication methods are supported.
Graphs and reports are shown in real-time at different levels of detail, thus helping to manage the testing process.
Download the free 30-day trial!
|
|
