Alter combination of 2 existing columns as a unique key

2 Message(s) by 2 Author(s) originally posted in mysql database

From: James

Date: Thursday, April 19, 2007

To alter 1 column to a unique key,

ALTER TABLE user MODIFY COLUMN id INT NOT NULL UNIQUE

<language> A {portable} {job control language}.

	["The UNIQUE Command Language - Portable Job Control",
	I.A. Newman, Proc DATAFAIR 73, 1973, pp. 353-357].

	(1994-11-22)

;

But how to set combination

1. <mathematics> A {set} containing a certain number of
	objects selected from another set.

	The number of combinations of r objects chosen from a set of n
	is

		n C r = n! / ((n-r)! r!)

	where "n C r" is normally with n and r as subscripts or as n
	above r in parentheses.

	See also {permutation}.

	2. <reduction> In the theory of {combinators}, a combination
	denotes an expression in which {function application} is the
	only operation.

	(1995-04-10)

of 2 columns as a unique key?
Individual keys aren't unique, but combination is.

TIA,
James

From: gordonb.4k7va

Date: Thursday, April 19, 2007

To alter 1 column to a unique key,
ALTER TABLE user MODIFY COLUMN id INT NOT NULL UNIQUE;
But how to set combination of 2 columns as a unique key?
Individual keys aren't unique, but combination is.

ALTER TABLE user ADD UNIQUE (id,domain);

That just adds an index. You might need to drop an old one first. Or not.

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