This special field sets sorting criteria for the records contained in a record set. Its usage is:
%sort: field1 field2 ...
Meaning that the desired order for the records will be determined by
the contents of the fields named in the %sort value. The
sorting is always done in ascending order, and there may be records
that lack the involved fields, i.e. the sorting
fields need not be mandatory.
It is an error to have more than one %sort field in the same
record descriptor, as only one field list can be used as sorting
criteria.
Consider for example that we want to keep the records in our inventory system ordered by entry date. We could achieve that by using the following record descriptor in the database:
%rec: Item %type: Date date %sort: Date Id: 1 Title: Staplers Date: 10 February 2011 Id: 2 Title: Ruler Pack 20 Date: 2 March 2009 ...
As you can see in the example above, the fact we use %sort in a
database does not mean that the database will be always physically
ordered. Unsorted record sets are not a data integrity
problem, and thus the diagnosis tools must not declare a recfile as
+invalid because of this. The utility recfix provides a way
+to physically order the fields in the file (see Invoking recfix).
On the other hand any program listing, presenting or processing data
extracted from the recfile must honor the %sort entry. For
example, when using the following recsel program in the
database above we would get the output sorted by date:
$ recsel inventory.rec Id: 2 Title: Ruler Pack 20 Date: 2 March 2009 Id: 1 Title: Staplers Date: 10 February 2011
The sorting of the selected field depends on its type:
It is possible to specify several fields as the sorting criteria. In that case the records are sorted using a lexicographic order. Consider for example the following unsorted database containing marks for several students:
%rec: Marks %type: Class enum A B C %type: Score real Name: Mr. One Class: C Score: 6.8 Name: Mr. Two Class: A Score: 6.8 Name: Mr. Three Class: B Score: 9.2 Name: Mr. Four Class: A Score: 2.1 Name: Mr. Five Class: C Score: 4
If we wanted to sort it by Class and by Score we would
insert a %sort special field in the descriptor, having:
%rec: Marks %type: Class enum A B C %type: Score real %sort: Class Score Name: Mr. Four Class: A Score: 2.1 Name: Mr. Two Class: A Score: 6.8 Name: Mr. Three Class: B Score: 9.2 Name: Mr. Five Class: C Score: 4 Name: Mr. One Class: C Score: 6.8
The order of the fields in the %sort field is
significant. If we reverse the order in the example above then we get
a different sorted set:
%rec: Marks %type: Class enum A B C %type: Score real %sort: Score Class Name: Mr. Four Class: A Score: 2.1 Name: Mr. Five Class: C Score: 4 Name: Mr. Two Class: A Score: 6.8 Name: Mr. One Class: C Score: 6.8 Name: Mr. Three Class: B Score: 9.2
In this last case, Mr. One comes after Mr. Two because the
class A
comes before the class B even though the score is the same (6.8).