On Wed, Nov 30, 2011 at 1:15 PM, Ian Clelland <clell...@gmail.com> wrote:
> On Wed, Nov 30, 2011 at 12:40 PM, Kääriäinen Anssi < > anssi.kaariai...@thl.fi> wrote: > >> """ >> Is this referring exclusively to natural, or user-specified primary key >> columns? Despite Luke's reference to nullable primary keys (are these even >> allowed by SQL?), a common idiom for copying objects is this: >> """ >> >> Not allowed by SQL specification, but many databases do allow them >> (source wikipedia). >> > > /me runs off to go correct Wikipedia ;) > > I checked the Wikipedia article on Primary Key first, and didn't see that, > but I did note this: > > A table can have at most one *primary key*, but more than one unique key. >> A primary key is a combination of columns which uniquely specify a row. It >> is a special case of unique keys. One difference is that primary keys have >> an implicit NOT NULL constraint while unique keys do not. > > > Also, this, from http://dev.mysql.com/doc/refman/5.5/en/create-table.html: > > A PRIMARY KEY is a unique index where all key columns must be defined as >> NOT NULL. If they are not explicitly declared as NOT NULL, MySQL declares >> them so implicitly (and silently). > > > And from > http://www.postgresql.org/docs/8.4/static/ddl-constraints.html#AEN2338: > > Technically, a primary key constraint is simply a combination of a unique >> constraint and a not-null constraint. > > > Oracle: > http://docs.oracle.com/cd/B19306_01/server.102/b14200/clauses002.htm: > > A primary key constraint combines a NOT NULL constraint and a unique >> constraint in a single declaration. That is, it prohibits multiple rows >> from having the same value in the same column or combination of columns and >> prohibits values from being null. > > > SQLite, of course, is special (http://www.sqlite.org/lang_createtable.html): According to the SQL standard, PRIMARY KEY should always imply NOT NULL. > Unfortunately, due to a long-standing coding oversight, this is not the > case in SQLite. Unless the column is an INTEGER PRIMARY KEY SQLite allows > NULL values in a PRIMARY KEY column. We could change SQLite to conform to > the standard (and we might do so in the future), but by the time the > oversight was discovered, SQLite was in such wide use that we feared > breaking legacy code if we fixed the problem. So for now we have chosen to > continue allowing NULLs in PRIMARY KEY columns. But, > Developers should be aware, however, that we may change SQLite to conform > to the SQL standard in future and should design new programs accordingly. I would consider Django 1.4+ to fall under the "new programs" umbrella :) -- Regards, Ian Clelland <clell...@gmail.com> -- You received this message because you are subscribed to the Google Groups "Django developers" group. To post to this group, send email to django-developers@googlegroups.com. To unsubscribe from this group, send email to django-developers+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/django-developers?hl=en.
