How do you differentiate between primary and secondary indexes? I'm not
familiar with the distinction.
Cal
http://www.calevans.com
-----Original Message-----
From: Thalis A. Kalfigopoulos [mailto:[EMAIL PROTECTED]]
Sent: Tuesday, March 27, 2001 10:55 AM
To: Cal Evans
Cc: Duke; [EMAIL PROTECTED]
Subject: RE: keywords in CREATE TABLE?
On Tue, 27 Mar 2001, Cal Evans wrote:
> Key = Creates an index
> Index = Same as above? (Not sure, the section of the manual I just glanced
> at seems to indicate that they are the same thing)
> PRIMARY KEY = Creates a primary key for the table. Each table should have
a
> primary key and it should be unique for that row in that table. (For more
on
> PK theory, checkout
>
http://www1.fatbrain.com/asp/BookInfo/BookInfo.asp?theisbn=0764504150&from=T
> GD352 )
> UNIQUE = Like primary key in that it creates a unique index on that field
> but it is not considered a primary key.
Hello Cal,
First: I don't think it is necessary that each table should have a primary
key.
Second: there seems to be some confusion between the concepts of primary key
and primary index.
>From the theory (at least the one I know) a primary index is NOT an index
on a primary key. Primary key is just the one of all the candidate keys that
was chosen by the DBA. Primary index on the other hand is the index whose
search key specifies the sequential order of the file.
So someone please bother to explain what happens exactly in Mysql. From what
I gather, when you specify a field(s) as PK, you just get another secondary
index on that field.
So we come to the conclusion that primary key is just another unique key
with a special name. Therefore the concept of primary index doesn't exist in
Mysql and the only flavor you have is secondary indexes either on unique or
non-unique keys.
Are the above conclusions correct?
regards,
thalis
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