Hi Shameem,

you have two ways.

one way:

std_id = fields.Char(string="Student ID", required=True)   

_sql_constraints = [('std_id_uniq', 'unique(std_id)', 'This ID already exists !')]

Another way using api:

mark1 = fields.Integer(string="Total Mark")

@api.constrains('mark1')   

def _check_mark1(self):
        if self.mark1 == 0:
            raise ValidationError("Please enter the marks !")

Also you can check any logical operations inside the constrains function.

Hope this may help you