#Create index on employees table with column employee_id in ascending order

db.employees.ensureIndex( { emp_id: 1 } )

In SQL:
create index on idx_emp on employees (emp_id);

# create index on employees table with column emp_id and age(desc order)

db.employees.ensureIndex( { emp_id: 1, age: -1 } )

In SQL:
create index on idx_emp1 on employees(emp_id,age desc);