add unique constraint to existing table column

Question

In mysql, when creating a table, you can append:

unique(username)

but i dont know how to alter a table to add that containt.

I was trying:

alter table Company constraint unique (username)

Answer 1

I mixed up the syntax.

alter table Company add constraint unique (username);

but this will fail if there are duplicates... So either drop and recreate if it is a sample db, or delete out duplicates.