[thelist] Multi-level data storage

rudy r937 at interlog.com
Tue Feb 27 16:21:37 CST 2001


> This is probably going to sound really dumb, but why are there two
> PRIMARY KEYs in the second table?

hi andrew

no such thing as a dumb question (okay, maybe "What's the number for 911?")

there's only one primary key in the second table, but it's a composite key,
consisting of two columns

thus

     1   503
     1   504
     1   505
     2   321
     2   503
     2   505
     3   503
     3   937

are all unique, even though taken individually, each column has duplicates

because it's a primary key, no set of any two numbers can be duplicated

> I would have thought that the LinkID would need to
> appear multiple times with each CatID that applied,

and so it can

there is an alternate way to do this, which is to create an auto_increment
primary key for this table, and then the above two columns can just be
carried along as data -- but this method is unnecessary if the database can
handle the first method (using a composite primary key)

helps?


rudy





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