Database Program Translation - Information Systems Reengineering, Integration and Normalization - page 193

The syntax for the algorithm Update is:

Relational Network

Exec SQL Update Table-T
set F1 = V1
and F2 = V2
and Fn = Vn
End-Exec.

Obtain first N k within IX k using ID k .
If return-code ≠ 0
error-exit
else
if A ≠ foreign key
Move V1 to F1
…..
Move Vn to Fn
Modify N k
else
if V ≠ null and value(A) = null
Find first N n within Ix n
Connect N k to N n
else
if V = null and value(A) ≠ null
Find first N k-1 within IX k-1
Disconnect N k from N k-1
else
if V ≠ null and value (A) ≠ null
Find first N k-1 within IX k-1
Find first N n within Ix n
Reconnect N k from N k-1 to N n.

Step 2. Processing network database data manipulation language.
The translated network database program is now ready for processing. From
the users' point of view, they are executing an embedded-SQL program. However,
from the system point of view, the embedded-SQL program has been translated
into a network database program, to access the network DBMS. Because of the
equivalent translated network DML statements (compared with the embedded-SQL
statements), the result of the translated network database program is the same as
the result of the embedded-SQL program. Such processing can be successful if ac-
complished by changing the execution environment, i.e., mapping the relational
schema of the embedded-SQL program to the network schema of the translated
network database program in the pre-process step 1 and adding secondary indices
to the translated network schema and the network database in the preprocess step 2.
On the other hand, even with the secondary indices added to the network database,
the existing network database program, after recompilation with the modified sec-
ondary-indices-add network schema, can still access the modified network database
as it did before the addition, as Fig. 5.7 shows (Fig. 5.7 is an extension of Fig. 5.6 ).
As a result, the modified network DBMS acts as a relational interface to relational
database program and as a network DBMS to the network database program. The