NULL values will propagate through an expression according to a few logical and quite reasonable rules.
Functions
The general rule is that functions return NULL when the parameters fall outside the range for which the function is defined.
Example:
|
asin(2) |
returns |
NULL |
|
log(-5) |
returns |
NULL |
|
round(A,0) |
returns |
NULL |
As a result of the above follows that functions generally return NULL when any of the parameters necessary for the evaluation are NULL.
Example:
|
sin(NULL) |
returns |
NULL |
|
chr(NULL) |
returns |
NULL |
|
if(NULL, A, B) |
returns |
NULL |
|
if(True, NULL, A) |
returns |
NULL |
|
if(True, A, NULL) |
returns |
A |
The exception to the second rule are logical functions testing for type.
Example:
|
isnull(NULL) |
returns |
True (-1) |
|
isnum(NULL) |
returns |
False (0) |
Arithmetic and string operators
If NULL is encountered on any side of these operators NULL is returned.
Example:
|
A |
+ |
NULL |
returns |
NULL |
|
A |
- |
NULL |
returns |
NULL |
|
A |
/ |
NULL |
returns |
NULL |
|
A |
* |
NULL |
returns |
NULL |
|
NULL |
/ |
A |
returns |
NULL |
|
0 |
/ |
NULL |
returns |
NULL |
|
0 |
* |
NULL |
returns |
NULL |
|
A |
& |
NULL |
returns |
A |
Relational operators
If NULL is encountered on any side of relational operators special rules apply.
Example:
|
NULL |
rel.op |
NULL |
returns |
NULL |
|
A |
<> |
NULL |
returns |
True (-1) |
|
A |
< |
NULL |
returns |
False (0) |
|
A |
<= |
NULL |
returns |
False (0) |
|
A |
= |
NULL |
returns |
False (0) |
|
A |
>= |
NULL |
returns |
False (0) |
|
A |
> |
NULL |
returns |
False (0) |
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