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From:Martijn Tonies Date:December 29 2004 10:32am
Subject:Re: unique key <-> primary key
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> >>  I have two tables, seemigly very similar setup; the primary key is the
> >>  combination of two columns. With mysqldump, however, the table
> >>  definition of the two tables looks different.
> >>
> >>  Mysqldump on table 1 says
> >>      ...
> >>      UNIQUE KEY HONstid (HONstid,HONname)
> >>  whereas on table 2 it says
> >>      ...
> >>      PRIMARY KEY  (IMstid,IMfnum)
> >>
> >>  What is the difference? Is there any?
> >
> >A table can have only 1 primary key, but multiple unique constraints.
>
> And columns in primary keys must be NOT NULL.  Columns in unique
> keys can be NULL (if they are NOT NULL, then the unique key is
> functionally the same as a primary key).

Ah yes Paul, thanks for adding that.

With regards,

Martijn Tonies
Database Workbench - developer tool for InterBase, Firebird, MySQL & MS SQL
Server
Upscene Productions
http://www.upscene.com

Thread
unique key <-> primary keyGaspar Bakos28 Dec
  • Re: unique key <-> primary keyMartijn Tonies28 Dec
    • Re: unique key <-> primary keyPaul DuBois28 Dec
      • Re: unique key <-> primary keyGaspar Bakos28 Dec
  • Re: unique key <-> primary keyMartijn Tonies29 Dec
  • Re: unique key <-> primary keyMartijn Tonies29 Dec