CREATE UNIQUE INDEX username ON users (username);
The CREATE UNIQUE INDEX command creates a unique index on a table (no duplicate values allowed)
Indexes are used to retrieve data from the database very fast. The users cannot see the indexes, they are just used to speed up searches/queries.
The following SQL creates an index named "uidx_pid" on the "PersonID" column in the "Persons" table:
CREATE UNIQUE INDEX uidx_pid
ON Persons (PersonID);