We should create an index if a column contains a wide range of values a column contains a large number of null values one or more columns are frequently used together in a WHERE clause or a join condition and the table is large and most queries are expected to retrieve less than 2-4 of the row.

Format of Create Index:

Create Index emp_last_name_idx ON emp (dept_id);

If you want to exforce uniqunes you should define a unique or primary key constraint in the table definition. then a unique index is created automatially.

Constraint emp_email_uk UNIQUE(email);