Is it possible to insert a row but only if a value does not already exist?

Question

Is it possible to insert a row, but only if one of the values already in the table does not exist?

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Answer 1

MySQL offers REPLACE INTO http://dev.mysql.com/doc/refman/5.0/en/replace.html:

REPLACE works exactly like INSERT, except that if an old row in the table has the same value as a new row for a PRIMARY KEY or a UNIQUE index, the old row is deleted before the new row is inserted.