Identifying Relation vs Non-Identifying?

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Dan - 01 Jul 2005 22:22 GMT

Hi,

Can someone explain to be me the difference to between an identifying
relation and a non-identifying relation? I came across these terms when
I recently started looking into using ERM tool.

TIA,
Dan

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Dan - 01 Jul 2005 22:26 GMT

> Hi,
>
[quoted text clipped - 4 lines]
> TIA,
> Dan

I should also state that the ERM tools I have looked at use these terms
for drawing a line between tables. I always thought a relation was a
table. So I am confused!

Dan

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Jan Hidders - 02 Jul 2005 00:27 GMT

> Can someone explain to be me the difference to between an identifying
> relation and a non-identifying relation? I came across these terms when
> I recently started looking into using ERM tool.

Don't you mean an "identifying relation*ship*"? Be careful not to
confuse the notions of relationship as used in ERDs and relation as used
in the Relational Model.

Loosely speaking an identifying relationship is a relationship that is
necessary to identify a certain entity that plays a role in it. Consider
for example the relationship "owns" between "Person" and "Dog" where
"Dog" only has attribute "name". In that case a dog is probably not
uniquely identified by just its name, but it will be by a combination of
name and owner. In some sense you could say that the relationship "owns"
is part of the key of "Dog". And that makes it an identifying
relationship. Also google for "weak entity".

Hope that helps,

-- Jan Hidders

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Dan - 05 Jul 2005 16:57 GMT

>> Can someone explain to be me the difference to between an identifying
>> relation and a non-identifying relation? I came across these terms
[quoted text clipped - 3 lines]
> confuse the notions of relationship as used in ERDs and relation as used
> in the Relational Model.

The 2 tools call it a "relation". Since it is for drawing a line
between tables (relations), I guess they should call it "relationship".

> Loosely speaking an identifying relationship is a relationship that is
> necessary to identify a certain entity that plays a role in it. Consider
[quoted text clipped - 4 lines]
> is part of the key of "Dog". And that makes it an identifying
> relationship. Also google for "weak entity".

Makes sense to me.

> Hope that helps,
>
> -- Jan Hidders

Thanks Jan.

Dan

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Dan - 05 Jul 2005 17:29 GMT

>>> Can someone explain to be me the difference to between an identifying
>>> relation and a non-identifying relation? I came across these terms
[quoted text clipped - 17 lines]
>
> Makes sense to me.

Can one just say that a weak entity is a tuple whose primary key relies
on the primary key of another entity? And that this type of
relationship and only this type is then defined as a weak relation?

Dan

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Jan Hidders - 05 Jul 2005 20:58 GMT

>>>> Can someone explain to be me the difference to between an
>>>> identifying relation and a non-identifying relation? [...]
[quoted text clipped - 6 lines]
> on the primary key of another entity? And that this type of
> relationship and only this type is then defined as a weak relation?

Yes. That is how you usually map such a construct in an ER diagram to
relations. The PK of the weak entity will consist of the local
identifying attrubutes plus the PK of the entity it depends upon. Things
can get a bit more complicated if the identifying relationship is a
many-to-many relationship, but that situation is very rare and some even
disallow it.

-- Jan Hidders

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dand@dhdursoassociates.com - 19 Aug 2005 08:19 GMT

Here's another cut at the definition of weak entity. An example might be
dependents which would not typically "stand alone" in an HR database without
employees. The PK of dependents would not necessarily contain the employee
PK, although there would be a FK to employees. Dependents, then would be a
weak entity, employees a strong entity. This relationship is
non-identifying.

An ID-Dependent weak entity on the other hand would need to include the PK
of the other entity. It cannot exist without the associated strong entity.
Example: dorm room and dorm. A dorm room BH12A is existence dependent on the
dorm BH. This is an identifying relationship. CASE tools often use this fact
to migrate the PK to the child table.

Signature

Dan D'Urso
-------------------
dh.durso1@d2associates.com
http://www.d2associates.com

> >>>> Can someone explain to be me the difference to between an
> >>>> identifying relation and a non-identifying relation? [...]
[quoted text clipped - 15 lines]
>
> -- Jan Hidders

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