Examples Of Illegal Links - Tandem ENFORM 058057 Reference Manual

Examples of Illegal Links

The following examples involve query specifications that use the sample data base
shown in the ENFORM User's Guide. The purpose of these examples is to show illegal
links, not to show useful queries. Each example includes a sketch of its links. The
following symbols are used for the sketches:
represents a one-directional link initiated by
a LINK OPTIONAL statement.
represents a two-directional link initiated by
either a LINK statement or a WHERE clause.
represents a two-directional link cancelled by
a LINK OPTIONAL statement.
The links in the following query specifications are illegal because they violate rule 1:
LINK region TO employee VIA regnum;
LINK employee.empnum TO OPTIONAL order.salesman;
LINK order TO region VIA regnum;
LIST regname, empname, ordernum;
A sketch of these links appears as follows:
region
In the preceding example, order (the record description on the right side of the LINK
OPTIONAL statement) is linked back to employee due to the transitive property of
links.
The query specifications in the following example violate rule 1. These query
specifications illustrate the fact that ENFORM evaluates a link established by a
WHERE clause in the same manner that it evaluates a link established by a LINK
statement:
LINK region TO employee VIA regnum;
LINK employee.empnum TO OPTIONAL order.salesman;
LIST regname, empname, ordernum
WHERE order.regnum = region.regnum;
A sketch of these links appears as follows:
region
058057 Tandem Computers Incorporated
LINKS and the LINK OPTIONAL Statement Rules
Review of Rules for the LINK OPTIONAL Statement
employee
order
employee
order
C–7