<s>
In	O
database	O
systems	O
,	O
a	O
propagation	B-Application
constraint	I-Application
"	O
details	O
what	O
should	O
happen	O
to	O
a	O
related	O
table	O
when	O
we	O
update	O
a	O
row	O
or	O
rows	O
of	O
a	O
target	O
table	O
"	O
(	O
Paul	O
Beynon-Davies	O
,	O
2004	O
,	O
p.108	O
)	O
.	O
</s>
<s>
Tables	O
are	O
linked	O
using	O
primary	B-Application
key	I-Application
to	O
foreign	B-Application
key	I-Application
relationships	O
.	O
</s>
<s>
It	O
is	O
possible	O
for	O
users	O
to	O
update	O
one	O
table	O
in	O
a	O
relationship	O
in	O
such	O
a	O
way	O
that	O
the	O
relationship	O
is	O
no	O
longer	O
consistent	O
and	O
this	O
is	O
known	O
as	O
breaking	O
referential	B-General_Concept
integrity	I-General_Concept
.	O
</s>
<s>
An	O
example	O
of	O
breaking	O
referential	B-General_Concept
integrity	I-General_Concept
:	O
if	O
a	O
table	O
of	O
employees	O
includes	O
a	O
department	O
number	O
for	O
'	O
Housewares	O
 '	O
which	O
is	O
a	O
foreign	B-Application
key	I-Application
to	O
a	O
table	O
of	O
departments	O
and	O
a	O
user	O
deletes	O
that	O
department	O
from	O
the	O
department	O
table	O
then	O
Housewares	O
employees	O
records	O
would	O
refer	O
to	O
a	O
non-existent	O
department	O
number	O
.	O
</s>
<s>
Propagation	B-Application
constraints	I-Application
are	O
methods	O
used	O
by	O
relational	B-Application
database	I-Application
management	I-Application
systems	I-Application
(	O
RDBMS	O
)	O
to	O
solve	O
this	O
problem	O
by	O
ensuring	O
that	O
relationships	O
between	O
tables	O
are	O
preserved	O
without	O
error	O
.	O
</s>
<s>
In	O
his	O
database	O
textbook	O
,	O
Beynon-Davies	O
explains	O
the	O
three	O
ways	O
that	O
RDBMS	O
handle	O
deletions	O
of	O
target	O
and	O
related	O
tuples	B-Application
:	O
</s>
<s>
Restricted	O
Delete	O
-	O
the	O
user	O
cannot	O
delete	O
the	O
target	O
row	O
until	O
all	O
rows	O
that	O
point	O
to	O
it	O
(	O
via	O
foreign	B-Application
keys	I-Application
)	O
have	O
been	O
deleted	O
.	O
</s>
<s>
Cascades	O
Delete	O
-	O
can	O
delete	O
the	O
target	O
row	O
and	O
all	O
rows	O
that	O
point	O
to	O
it	O
(	O
via	O
foreign	B-Application
keys	I-Application
)	O
are	O
also	O
deleted	O
.	O
</s>
<s>
Nullifies	O
Delete	O
-	O
can	O
delete	O
the	O
target	O
row	O
and	O
all	O
foreign	B-Application
keys	I-Application
(	O
pointing	O
to	O
it	O
)	O
are	O
set	O
to	O
null	B-Language
.	O
</s>
<s>
In	O
this	O
case	O
,	O
after	O
removing	O
the	O
housewares	O
department	O
,	O
employees	O
who	O
worked	O
in	O
this	O
department	O
would	O
have	O
a	O
NULL	B-Language
(	O
unknown	O
)	O
value	O
for	O
their	O
department	O
.	O
</s>
