course name  all ready exits

_sql_constraints = [ ('course_name',

                                     'UNIQUE (course_name)',

                                    'Course all already exists'), ]

This code is for identify unique student id.If any person write duplicate student which is already in the database it gives an error like "Student ID is already exists...!"

_sql_constraints = [

                 ('stud_id_unique', 'unique (stud_id)', 'Student ID is already exists...!')

]

_sql_constraints = [

('email_uniq', 'unique(email)', 'Email id is unique change your custom email id'),

]

@api.constrains('email')

def validate_email(self):

for obj in self:

if re.match("^.+\\@(\\[?)[a-zA-Z0-9\\-\\.]+\\.([a-zA-Z]{2,3}|[0-9]{1,3})(\\]?)$", obj.email) == None:

raise ValidationError("Please Provide valid Email Address: %s" % obj.email)

return True

Both constraints are working fine in my module.

you can use @api.constrains too

Hi,

You may refer any addon in Odoo 9.0. 

For eg, in account module, goto account >> models >> account.py

_sql_constraints = [
        ('code_company_uniq', 'unique (code,company_id)', 'The code of the account must be unique per company !')
    ]

You can refer the usage.