unique in aggregate section

Author: ginsbach

docs/language/ql-spec/language.rst

@@ -1058,6 +1058,7 @@ An aggregation can be written in one of two forms:
 
    aggregation ::= aggid ("[" expr "]")? "(" (var_decls)? ("|" (formula)? ("|" as_exprs ("order" "by" aggorderbys)?)?)? ")"
                |   aggid ("[" expr "]")? "(" as_exprs ("order" "by" aggorderbys)? ")"
+               |   "unique" "(" var_decls "|" (formula)? ("|" as_exprs)? ")"
 
    aggid ::= "avg" | "concat" | "count" | "max" | "min" | "rank" | "strictconcat" | "strictcount" | "strictsum" | "sum"
 
@@ -1098,7 +1099,7 @@ The typing environment for ordering directives is obtained by taking the typing
 
 The number and types of the aggregation expressions are restricted as follows:
 
--  A ``max``, ``min`` or ``rank`` aggregation must have a single expression.
+-  A ``max``, ``min``, ``rank`` or ``unique`` aggregation must have a single expression.
 -  The type of the expression in a ``max``, ``min`` or ``rank`` aggregation without an ordering directive expression must be an orderable type.
 -  A ``count`` or ``strictcount`` aggregation must not have an expression.
 -  A ``sum``, ``strictsum`` or ``avg`` aggregation must have a single aggregation expression, which must have a type which is a subtype of ``float``.
@@ -1140,6 +1141,8 @@ The values of the aggregation expression are given by applying the aggregation f
 
 -  If the aggregation id is ``strictconcat``, then the result is the same as for ``concat`` except in the case where there are no aggregation tuples in which case the aggregation has no value.
 
+ -  If the aggregation id is ``unique``, then the result is the the value of the aggregation variable if there is precisely one such value. Otherwise, the aggregation has no value.
+
 Any
 ~~~