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is it possible to create a primary key in a existing table on a column that have repeated value? I want is previous record not validate but new record will validate with this.Is it possible in mysql. I know it is possible in Oracle (here is an example) but don’t have idea about mysql.
The link you posted as a comment to Nerd-Herd’s answer uses deferred constraints. Those constraints are checked at the end of the transaction rather than at the time the statement is executed.
MySQL does not support deferred constraints
If you absolutely need deferred constraints and want to stick with an open source database you will need to migrate to PostgreSQL.