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IBM's online
SQL Reference manual,
specifically the
CREATE TABLE statement,
says that a table can have up to 500 or up to 1012 columns (depending
on the byte size of each row).
For a primary key,
"The number of identified columns must
not exceed 16 and the sum of their stored lengths must
not exceed 1024. The length of any individual column must
not exceed 255 bytes."
Perhaps that's where you got the 255 number from?
In any case, 16 seems like a very workable limit
on the number of primary key columns.
If you have a table design that requires more, then
I would take another look at it.
Remember, the purpose of the primary
key is to ensure entity integrity (i.e. uniqueness of rows),
so if you need more than 16 columns to do that,
perhaps you should look into using a surrogate key.
See my previous answer
Using generated numbers for primary keys (10 June 2002).
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