Sql-server – Ensuring Uniqueness Across Columns

I have modified ypercube's solution a little bit, so that both source and destination are not null, as is guaranteed by the original design. My CHECK constraints are commented out, as they apparently will not work in MySQL. I kept them as comments because they document my intent, and they will work on other RDBMS.

CREATE TABLE PointType
( PointTypeID tinyint unsigned NOT NULL,
  TypeDescription CHAR(20) NOT NULL,
  PRIMARY KEY (PointTypeID),
  UNIQUE (TypeDescription)
) ;

INSERT INTO PointType 
  (PointTypeID, TypeDescription)
VALUES
  (1, 'Source'), 
  (2, 'Destination') ;

CREATE TABLE PointUsageQuota
( RouteID int unsigned NOT NULL,
  PointTypeID tinyint unsigned NOT NULL,
  PointID smallint(5) unsigned NOT NULL,
  PRIMARY KEY (PointID),     -- and this is what all the fuss is about
  UNIQUE (RouteID , PointID, PointTypeID), -- target for the foreign keys
  FOREIGN KEY (PointTypeID)
    REFERENCES PointType (PointTypeID)
) ;

CREATE TABLE Route
( RouteID int unsigned NOT NULL,
  SourcePointID smallint(5) unsigned NOT NULL,
  SourceTypeID tinyint unsigned NOT NULL, 
      -- CHECK(SourceTypeID = 1),
  FOREIGN KEY (RouteID , SourcePointID, SourceTypeID) 
    REFERENCES PointUsageQuota (RouteID , PointID, PointTypeID) ,
  DestinationPointID smallint(5) unsigned NOT NULL,
  DestinationTypeID tinyint unsigned NOT NULL, 
      -- CHECK(DestinationTypeID = 2),
  FOREIGN KEY (RouteID , DestinationPointID, DestinationTypeID) 
    REFERENCES PointUsageQuota (RouteID , PointID, PointTypeID) ,
  --  other fields
  PRIMARY KEY (RouteID)
) ;

Testing:

insert into PointUsageQuota
values(1,1,666),(1,2,354);

INSERT INTO Route VALUES (1, 666, 1, 354, 2);
-- this fails:
INSERT INTO Route VALUES (2, 666, 1, 354, 2);
-- this fails too:
INSERT INTO Route VALUES (2, 354, 1, 666, 2);

Use a unique constraint (unique index), this is exactly what they're for.

The difference will be in your code. If you use the unique constraint method, you need to catch the error that the dbms will throw on duplicate entries, but if you check for the value explicitly first, you won't need to catch an error.

However, if you don't use the unique constraint method, you're not really restricting the duplication of the data at the database level, allowing the data to be corrupted easily by another application or even a maintenance script being run by a careless developer or dba.