so do a seach on the web for 'surrogate keys': hint, a sequence, primary key column.
alex b. wrote:
> Doug McNaught wrote:
>
>> "alex b." <mailinglists1@gmx.de> writes:
>>
>>
>>> CREATE TABLE tmp3 (
>>>
>>> datname VARCHAR(255),
>>> title VARCHAR(100),
>>> artist VARCHAR(100),
>>> album VARCHAR(100),
>>> mp3checksum CHAR(32),
>>> CONSTRAINT md5
>>> FOREIGN KEY (mp3checksum)
>>> REFERENCES tcontent (mp3checksum)
>>> ON DELETE CASCADE
>>> );
>>>
>>> NOTICE: CREATE TABLE will create implicit trigger(s) for FOREIGN KEY
>>> check(s)
>>> ERROR: UNIQUE constraint matching given keys for referenced table
>>> "tcontent" not found
>>>
>>> what am I doing wrong here?
>>
>>
>>
>> Column "mp3checksum" of table "tcontent" needs a UNIQUE
>> constraint--does it have one?
>
>
> no it doesn't - because it is possible that the same exact file might be
> existent on multiple CDs...
> and therefore "mp3checksum" cannot have a UNIQUE constraint.
> am I right?
>
>
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