How do I specify unique constraint for multiple columns in MySQL?

Question

I have a table:

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

Now I want to make the columns user

Answer 1

MySql 5 or higher behaves like this (I've just tested):

• you can define unique constraints involving nullable columns. Say you define a constraint unique (A, B) where A is not nullable but B is
• when evaluating such a constraint you can have (A, null) as many times you want (same A value!)
• you can only have one (A, not null B) pair

Example: PRODUCT_NAME, PRODUCT_VERSION 'glass', null 'glass', null 'wine', 1

Now if you try to insert ('wine' 1) again it will report a constraint violation Hope this helps