MySQL - Make an existing Field Unique

问题

I have an already existing table with a field that should be unique but is not. I only know this because an entry was made into the table that had the same value as another, already existing, entry and this caused problems.

How do I make this field only accept unique values?

回答1:

ALTER IGNORE TABLE mytbl ADD UNIQUE (columnName);

For MySQL 5.7.4 or later:

ALTER TABLE mytbl ADD UNIQUE (columnName);

As of MySQL 5.7.4, the IGNORE clause for ALTER TABLE is removed and its use produces an error.

So, make sure to remove duplicate entries first as IGNORE keyword is no longer supported.

Reference

回答2:

Just write this query in your db phpmyadmin.

ALTER TABLE TableName ADD UNIQUE (FieldName)

Eg: ALTER TABLE user ADD UNIQUE (email)

回答3:

If you also want to name the constraint, use this:

ALTER TABLE myTable
  ADD CONSTRAINT constraintName 
    UNIQUE (columnName);

回答4:

CREATE UNIQUE INDEX foo ON table_name (field_name)

You have to remove duplicate values on that column before executes that sql. Any existing duplicate value on that column will lead you to mysql error 1062

回答5:

The easiest and fastest way would be with phpmyadmin structure table.

There it's in Russian language but in English Version should be the same. Just click Unique button. Also from there you can make your columns PRIMARY or DELETE.

回答6:

ALTER IGNORE TABLE mytbl ADD UNIQUE (columnName);

is the right answer

the insert part

INSERT IGNORE INTO mytable ....

回答7:

This code is to solve our problem to set unique key for existing table

alter ignore table ioni_groups add unique (group_name);

来源：https://stackoverflow.com/questions/5038040/mysql-make-an-existing-field-unique

标签

mysql

unique-constraint