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DEPT
JOB
DnoDname Location
Jid
TitleSalary
Works-in
assign
EMP
Eno EnameBirthyr
qual-job
qual-emp
QUAL
Fig. 5.3
A sample network schema for decompilation
For example, Katz and Wong (
1982
) designed a decompilation method that pro-
ceeded in two phases. The first phase is analysis. During this phase, a network
database program is partitioned into blocks of statements for which an entry can
only occur at the first statement. The user then seeks to group together a sequence
of FIND statements that reference the same logically definable set of records, and
to aggregate these sets whenever possible. The result is the mapping of a DML
program into access path expressions. The second phase is embedding, where the
access path expression is mapped into a relational query and interfaced with the
original program.
For instance, consider a program that finds the departments for which accoun-
tants born after 1950 are assigned, using the following network schema in Fig.
5.3
.
The corresponding relational schema is:
Relation DEPT (
Dno
, Dname, Location)
Relation JOB (
Jid
, Title, Salary)
Relation EMP (
Eno
, Ename, Birthyr, Dno, Jid)
Relation QUAL (*
Eno
, *
Jid
)
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