Re: What is a tuple?

Well, maybe just to keep things fun, I'll confuse the heck out of
all you guys.

To be pedantic, "relation" and "tuple" are mathmatical terms used
in relational algebra; "table" and "row" are references to entities
in physical storage.

A relation cannot contain duplicate tuples, but a table can contain
duplicate rows. So let's look at the following:

key | value
--------+---------------------
1 | one
2 | two
2 | two

This can be a table, but not a relation. The relation wouldn't have
that last row in it.

But in fact, it gets more interesting. Tables don't actually
correspond to relations; they correspond to _relvars_, or variables
that hold relations. Given the following table "foo":

SELECT * FROM foo

key | value
--------+---------------------
1 | one
2 | two

DELETE FROM foo WHERE key = 2
SELECT * FROM foo

key | value
--------+---------------------
1 | one

The two relations you see above are obviously not equal. They're
two different relations. When you deleted that row/tuple, you
effectively assigned a new relation to the relvar "foo".

There's more like this, but it can be most effectively appreciated by
reading C. J. Date's book _An Introduction to Database Systems_.

cjs
--
Curt Sampson <cjs(at)cynic(dot)net> +81 90 7737 2974 http://www.netbsd.org
Don't you know, in this new Dark Age, we're all light. --XTC