Monday, 2 November 2015

SQL ISNULL(), NVL(), IFNULL() and COALESCE() Functions

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:

MS Access

SELECT ProductName,UnitPrice*(UnitsInStock+IIF(ISNULL(UnitsOnOrder),0,UnitsOnOrder))
FROM Products
SQL Server

SELECT ProductName,UnitPrice*(UnitsInStock+ISNULL(UnitsOnOrder,0))
FROM Products

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 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

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