Within the Message Data Model (MDM) there are many optional fields. For example,
body.html might be missing if an email message wasn't encoded with HTML. Within Message Query Language (MQL),
null values are handled carefully and automatically to maintain consistent behavior.
Rule of thumb for how MQL handles missing/null values
If an expression can't be evaluated because one of the required arguments is missing (
null), then the result is also
To see how the boolean operations
notare performed in the presence of
null, see boolean-logic-with-null
For example, a rule might consist of this logic
like(body.html.raw, "*cloudtracking.gif*). What happens if
body.html is missing? What if
body.html.raw is missing?
This is easy to test by substituting
null for the value. If a field is missing, like
body.html, then any subfields will always be missing, like
To test the behavior ahead of time, substitute
null in its place.
like(null, "*cloudtracking.gif*") -> null # if body.html is missing: like(body.html.raw, "*cloudtracking.gif*") -> null
If either side of a comparison (e.g. ,
==, ...) is
null, the full comparison will evaluate as
null, even if both sides are null.
# if body.html is missing: length(body.html.raw) > 4096 -> null null == null -> null null != null -> null
To explicitly test if a value is missing, use
is null or
is not null syntax, which will always return
# if body.html is missing body.html is null -> true body.html is not null -> false # if body.html is present body.html is null -> false body.html is not null -> true
null boolean adds another level of logic to the boolean operators
not. There are certain logical rules that must be maintained for consistency. For example,
not (x == y) is the same as
x != y. Similarly,
not (x < y) is always equivalent to the expression
x >= y. Because this is always true when the values are not
null, it must remain true if a
null is present.
null is never implicitly converted to
false when performing boolean logic. Let's see what happens if we explicitly check for
is not null for the below expression. Note how it changes the result so that it no longer returns
not (length(body.html.raw) > 4096)
not (length(body.html.raw) > 4096) => not (7000 > 4096) => not (true) => false not (body.html is not null and (length(body.html.raw) > 4096)) => not (true and (7000 > 4096)) => not (true and true) => not (true) => false
not (length(body.html.raw) > 4096) => not (600 > 4096) => not (false) => true not (body.html is not null and (length(body.html.raw) > 4096)) => not (true and (600 > 4096)) => not (true and false) => not (false) => true
not (length(body.html.raw) > 4096) => not (null > 4096) => not (null) => null not (body.html is not null and (length(body.html.raw) > 4096)) => not (false and null) => not (false and null) => not (false) => true
The truth tables for boolean operations are below. Both
or are associative, meaning that
x and y is equivalent to
y and x.
x and y
x or y
x of (... N clauses ...)
The table from
of is derived from the behavior of
or and is fully compatible. That is,
2 of (a, b, c) is always logically equivalent to
(a and b) or (a and c) or (b and c).
|more than ||false|
Updated 2 months ago