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RM and definition of relations/tuples

From: Martin Zapf <Martin_Zapf_at_gmx.net>
Date: Wed, 23 Nov 2005 14:28:32 +0100
Message-ID: <dm1qq0$utl$1_at_online.de> 





I have a question to the Relational Model and the definition of 
relations and tuples.




I learned the following definitions:



A relation schema R is a set of Attributes R={A_1,A_2,...,A_n}
Each Attribute A has a domain dom(A)



Here comes the problem, there are two definitions for relations/tuples



    
  
  1.   defintion:
	




	A relation r for schema R is a mathematical relation (cartesian
	product) over the
	domains from the attributes of R.
	So r:=dom(A_1)xdom(A_2)x...xdom(A_n)
	A tuple is an element of r.





2. definition


	A relation r for schem R is a set of tuples.
	A tuple t is a function
	t: R -> Union (dom(A_1),dom(A_2),...,dom(A_1))
	t maps each Attribute of R to an value of its domain.
	So a relation is a set of functions, which are called tuples.






I noticed, that the difference between this two definition is that 
definition 1 forces a certain order for the Attributes and the values 
for them in the tuples.


The 2. definition doesnt need any order for the attributes.




Im quite confussed, is there a "better" definition or should you always 
use both?
Received on Wed Nov 23 2005 - 14:28:32 CET












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