Microsoft Acess 2007: Primary Key
Hi,
1) I am studying the Microsoft Access 2007
2) There are some objects of Microsoft Access, such as Table, Form, Query, etc
3) I saw a term called "Primary key" at a Table
4) My questions:
First: What is "Primary Key"? (Pls explain with your own wording and understanding; i do not need the links)
Second: This Primary key is ONLY for a Table? (or other OBJECTS such as Form, Query, Report, etc ........ALSO have a Primary key?)
Third: Other things related to Primary keys? please explain
5) Thank you
tjie
1) I am studying the Microsoft Access 2007
2) There are some objects of Microsoft Access, such as Table, Form, Query, etc
3) I saw a term called "Primary key" at a Table
4) My questions:
First: What is "Primary Key"? (Pls explain with your own wording and understanding; i do not need the links)
Second: This Primary key is ONLY for a Table? (or other OBJECTS such as Form, Query, Report, etc ........ALSO have a Primary key?)
Third: Other things related to Primary keys? please explain
5) Thank you
tjie
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So tjie ... You stated "Pls explain with your own wording and understanding; i do not need the links"
What happened ?
mx
What happened ?
mx
ASKER
Hey mx,
1) Yes, it is correct
2) I read Gary's personal wording (not the link) and he is explaining earlier ...
3) Thank you for both of you
tjie
1) Yes, it is correct
2) I read Gary's personal wording (not the link) and he is explaining earlier ...
3) Thank you for both of you
tjie
Gary's wording is a *direct* copy from the first paragraph of the article, sorry.
mx
mx
ASKER
mx,
1) Sorry, i do not check it
2) I owe you one
3) Thanks for the correction
tjie
1) Sorry, i do not check it
2) I owe you one
3) Thanks for the correction
tjie
The definition given isn't quite sufficient because a primary key is actually supposed to be a candidate key and a candidate key is not only required to uniquely identify a tuple (row) it must also be irreducibly unique - ie, if you remove any attribute from the key it will no longer be unique.
A set of attributes that is unique (ie. "uniquely identifies a row") is called a superkey.
A candidate key is a minimal superkey.
A primary key is a candidate key.
The minimality requirement is therefore a distinguishing factor of a primary key, not only its uniqueness. This is important because dependency theory and the normalization process attache signifcance to identifying candidate keys and the dependencies on them.
A set of attributes that is unique (ie. "uniquely identifies a row") is called a superkey.
A candidate key is a minimal superkey.
A primary key is a candidate key.
The minimality requirement is therefore a distinguishing factor of a primary key, not only its uniqueness. This is important because dependency theory and the normalization process attache signifcance to identifying candidate keys and the dependencies on them.
mx