docs(validations): a range rule compares in SAS's own order

A range rule keys both the cell value and its own value into the order SAS uses
for a numeric variable: every missing below every non-missing value, and the
missing values ordered ._ then the regular missing then .A through .Z. So a range
can be written in special missings and mean what SAS means by it - MINVAL .A with
MAXVAL .C accepts .B and rejects .D - and the numeric cases follow from the same
order: a missing fails a numeric MINVAL and passes a numeric MAXVAL, and a number
sits above every missing.

Replaces the "a range rule ignores a missing value" wording, which described the
previous take's behaviour rather than SAS's.

docs/dcc-validations.md

@@ -56,8 +56,8 @@ The rules treat a special missing as a value for some checks and as a missing va
 |Rule|What happens to a special missing|
 |---|---|
 |NOTNULL|Fails.  A special missing is a missing value, so a NOT NULL (or primary key) constraint rejects it, and a real SAS NOT NULL constraint on the target table rejects it too.|
-|MINVAL|Passes.  A missing value is not a number, so a minimum does not apply to it - use NOTNULL if the column must be populated.  A real number below the minimum still fails.|
+|MINVAL|Fails if the minimum is a number - a missing sorts below every number, so it is below any numeric floor.  A minimum that is itself a special missing is compared in the order the missing values have among themselves, and can accept one.|
-|MAXVAL|Passes, for the same reason.  A real number above the maximum still fails.|
+|MAXVAL|Passes - a missing sorts below every number, so it is below any numeric ceiling.  A ceiling that is itself a special missing is compared in the missing values' own order, and can reject one.|
 |CASE|Not applicable - it is a character rule.|
 |ROUND|Ignored - ROUND only rounds values that are numbers, so a special missing is left as typed.|
 |HARDREGEX / SOFTREGEX|Checked against the pattern like any other value.  Unlike blanks and the plain `.`, special missings are **not** exempt.  See [Regex Rules](#regex-rules).|
@@ -65,11 +65,13 @@ The rules treat a special missing as a value for some checks and as a missing va
 |HARDSELECT|Supports special missings - the value is matched against the dropdown list like any other value.|
 |HARDFORMULA / SOFTFORMULA|The formula returns `#VALUE!` for that row instead of a number, and that is what gets submitted.|
 
-In short, a special missing is rejected by NOTNULL (and by a primary key column), and a formula returns `#VALUE!` for it.  It is fine in a column that carries a range rule, a regex, a dropdown, or ROUND.
+In short, a special missing is rejected by NOTNULL (and by a primary key column), by a MINVAL whose floor is a number, and a formula returns `#VALUE!` for it.  It is fine in a column that carries a MAXVAL, a regex, a dropdown, or ROUND - and it can satisfy a MINVAL when the floor is expressed as a special missing.
+
+A range rule compares in the order SAS itself uses, so a range can be written in special missings and mean what SAS would mean by it.  That order puts every missing below every non-missing value, and orders the missing values as `._`, then the regular missing, then `.A` through `.Z`.  So `MINVAL .A` with `MAXVAL .C` accepts `.B` and rejects `.D`, and a blank - the regular missing - fails a floor of `.A` because it sorts below it.  A number sorts above every missing, so it passes a floor of `.A` and fails a ceiling of `.C`.
 
 A primary key column is treated as NOT NULL whether or not the target table carries a physical constraint, and whether or not MPE_VALIDATIONS has a NOTNULL rule for it - a key identifies the row, so a blank and a special missing are both rejected there.
 
-One related gotcha: a range rule's own value must be a number.  The rule value is compared as a number, so a special missing there leaves nothing for a real value to pass: put one in `MINVAL` and every number in the column fails; put one in `MAXVAL` and every number fails too.
+A range rule's own value is read as a number when it looks like one and as a missing value when it is a special missing.  A value that is neither - a typo such as `..` or `AB` - satisfies nothing, so every cell in the column fails until the rule is corrected.
 
 ## Formula Rules