Mysql – How to specify unique constraint for multiple columns in MySQL

composite-keydatabase-tableMySQLunique-constraint

I have a table:

table votes (
    id,
    user,
    email,
    address,
    primary key(id),
);

Now I want to make the columns user, email, address unique (together).

How do I do this in MySql?

Of course the example is just… an example. So please don't worry about the semantics.

Best Answer

To add a unique constraint, you need to use two components:

ALTER TABLE - to change the table schema and,

ADD UNIQUE - to add the unique constraint.

You then can define your new unique key with the format 'name'('column1', 'column2'...)

So for your particular issue, you could use this command:

ALTER TABLE `votes` ADD UNIQUE `unique_index`(`user`, `email`, `address`);

Related Solutions

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You can use GROUP_CONCAT:

SELECT person_id,
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FROM peoples_hobbies
GROUP BY person_id;

As Ludwig stated in his comment, you can add the DISTINCT operator to avoid duplicates:

SELECT person_id,
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FROM peoples_hobbies
GROUP BY person_id;

As Jan stated in their comment, you can also sort the values before imploding it using ORDER BY:

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