ISNULL (SQL)

A function that tests for NULL and returns the appropriate expression.

Synopsis

ISNULL(check-expression,replace-expression)

Arguments

Argument	Description
check-expression	The expression to be evaluated.
replace-expression	An expression that is returned if check-expression is NULL.

ISNULL returns the same data type as check-expression.

Description

ISNULL evaluates check-expression and returns one of two values:

If check-expression is NULL, replace-expression is returned.
If check-expression is not NULL, check-expression is returned.

The data type of replace-expression should be compatible with the data type of check-expression.

The collation of the resulting expression is determined by the first argument that is not of a collation type that may be coerced into the collation of the other. A collation that may be coerced into a different collation is called “coercible.” Possible scenarios are as follows:

The collations of check-expression and replace-expression are not coercible: The output collation is the collation of check-expression.
The collation of check-expression is not coercible, but replace-expression is coercible: The output collation is the collation of check-expression.
The collation of check-expression is coercible, but replace-expression is not coercible: The output collation is the collation of replace-expression.
The collations of check-expression and replace-expression are coercible: The output collation is coercible.

Note that the ISNULL function is the same as the NVL function, which is provided for Oracle compatibility.

Refer to NULL for further details on NULL handling.

DATE and TIME Display Conversion

Some check-expression data types require conversion from Logical mode to ODBC mode or Display mode, such as the DATE and TIME data types. If the replace-expression value is not the same data type, this value cannot be converted in ODBC mode or Display mode, and an SQLCODE error is generated: -146 for DATE data type; -147 for TIME data type. For example, ISNULL(DOB,'nodate') cannot be executed in ODBC mode or Display mode; it issues an SQLCODE -146 error with the %msg Error: 'nodate' is an invalid ODBC/JDBC Date value or Error: 'nodate' is an invalid DISPLAY Date value. To execute this statement in ODBC mode or Display mode, you must CAST the value as the appropriate data type: ISNULL(DOB,CAST('nodate' as DATE)). This results in a date 0, which displays as 1840-12-31.

NULL Handling Functions Compared

The following table shows the various SQL comparison functions. Each function returns one value if the logical comparison tests True (A same as B) and another value if the logical comparison tests False (A not same as B). These functions allow you to perform NULL logical comparisons. You cannot specify NULL in an actual equality (or non-equality) condition comparison.

SQL Function	Comparison Test	Return Value
ISNULL(ex1,ex2)	ex1 = NULL	True returns ex2 False returns ex1
IFNULL(ex1,ex2) [two-argument form]	ex1 = NULL	True returns ex2 False returns NULL
IFNULL(ex1,ex2,ex3) [three-argument form]	ex1 = NULL	True returns ex2 False returns ex3
{fn IFNULL(ex1,ex2)}	ex1 = NULL	True returns ex2 False returns ex1
NVL(ex1,ex2)	ex1 = NULL	True returns ex2 False returns ex1
NULLIF(ex1,ex2)	ex1 = ex2	True returns NULL False returns ex1
COALESCE(ex1,ex2,...)	ex = NULL for each argument	True tests next ex argument. If all ex arguments are True (NULL), returns NULL. False returns ex

Examples

In the following example, the first ISNULL returns the second expression (99) because the first expression is NULL. The second ISNULL returns the first expression (33) because the first expression is not NULL:

SELECT ISNULL(NULL,99) AS IsNullT,ISNULL(33,99) AS IsNullF

The following example returns the string 'No Preference' if FavoriteColors is NULL; otherwise, it returns the value of FavoriteColors:

SELECT Name,
ISNULL(FavoriteColors,'No Preference') AS ColorChoice
FROM Sample.Person

Compare the behavior of ISNULL with IFNULL:

SELECT Name,
IFNULL(FavoriteColors,'No Preference') AS ColorChoice
FROM Sample.Person