Okay. First of all can thanks for your answer. 

I understand Sql constraints.

What are the exact differences between _constraints & @api_constraints? I can speculate on the differences 

A brief but reply would be most helpful. 

Also I tried searching for api constraints. Could not find anything useful. Can you point me in the right direction?

Thanks once again

There is several ways to achieve that, please check v8.0 documentation or v7.0 documentation selon your needs. possible options are "_constraints", "_sql_constraints" and "@api.constrains" (the last option is v8 speciphic and it replaces v7 _constraints, which is deprecated in v8 ). 

EXAMPLES:

• _sql_constraints example of usage in "stock.py":
  
  _sql_constraints = [
          ('warehouse_name_uniq', 'unique(name, company_id)', 'The name of the warehouse must be unique per company!'),
          ('warehouse_code_uniq', 'unique(code, company_id)', 'The code of the warehouse must be unique per company!'),
      ]

• @api.constrain example of usage in "event.py":

      @api.one
      @api.constrains('event_id', 'state', 'nb_register')
      def _check_seats_limit(self):
          if self.event_id.seats_max and \
              self.event_id.seats_available < (self.nb_register if self.state == 'draft' else 0):
                  raise Warning(_('No more available seats.'))
   
• _constraints example of usage in "stock.py":

      def _check_uom(self, cr, uid, ids, context=None):
          for move in self.browse(cr, uid, ids, context=context):
              if move.product_id.uom_id.category_id.id != move.product_uom.category_id.id:
                  return False
          return True

      _constraints = [
          (_check_uom,
              'You try to move a product using a UoM that is not compatible with the UoM of the product moved. Please use an UoM in the same UoM category.',
              ['product_uom']),
      ]

The code in "_constraints" example above may be rewritten with "@api.constrains" as follows:

    @api.one
    @api.constrains('product_uom')
    def _check_uom(self):
        if self.product_id.uom_id.category_id.id != self.product_uom.category_id.id:
            raise Warning(_('You try to move a product using a UoM that is not compatible with the UoM of the product moved. Please use an UoM in the same UoM category.'))

_constraints and @api.constrains are the same concept and they allow to validate field in any sense (not only for uniqueness) using python function (as _check_uom - in examples above). _constraints is still supported but deprecated in v8, whereas @api.constrains makes part of new v8 api as its replacement

Constraints are applied only using ORM methods i.e. only in code and it does not affect the table whereas _sql_constraints set the rule at table level

In Odoo 7:
_sql_constraints and _constraints arguments were used

In Odoo 8:
_sql_constraints and @api.constraints arguments are used in v8.