Database Reference
In-Depth Information
Fig. 6.1
Relational DBMS
on. In order to provide clear separation of database manipulation, the host lan-
guage and embedded language are loosely coupled. The embedded formulas in
the original general-purpose programming language are usually preceded by the
statement 'EXEC...' like in SQL; the DBRM can be regarded as nothing more
than the “pure” embedded formulas. It does not consist of executable code. We
replace a standard SQL by
T
RE
terms (internal subalgebras of
Σ
RE
in Defini-
tion
31
) in the following way (see Table
6.1
). In the table,
1.
t
S
is a SPRJU (
Σ
R
)term;
2.
t
D
=
'
r
MINUS (
r
WHERE
C
)';
3.
t
I
=
'
r
UNION
t
1
', where
t
1
is an SPRJU term
t
S
=
'SELECT...'incase(ii);
t
1
=
r
otherwise (with
ar(r
)
=
ar(r)
, and
r
is a relational symbol for a
one-tuple relations obtained from the 'list of values' by substitution of each
value in this list with a fresh new variable and by adding NULL values for the
attributes of
r
which are not specified in the list
S
). The assignment of values
to these variables will be done during the execution of the program
P
when
