Designing a friendships database structure: Should I use a multivalued column?

-- You should determine which are the most fitting 
-- data types and sizes for all the table columns 
-- depending on your business context characteristics.

-- At the physical level, you should make accurate tests 
-- to define the mostconvenient INDEX strategies based on 
-- the pertinent query tendencies.

-- As one would expect, you are free to make use of 
-- your preferred (or required) naming conventions. 

CREATE TABLE UserProfile ( -- Represents an independent entity type.
    UserId          INT      NOT NULL,
    FirstName       CHAR(30) NOT NULL,
    LastName        CHAR(30) NOT NULL,
    BirthDate       DATE     NOT NULL,
    GenderCode      CHAR(3)  NOT NULL,
    Username        CHAR(20) NOT NULL,
    CreatedDateTime DATETIME NOT NULL,
    --
    CONSTRAINT UserProfile_PK  PRIMARY KEY (UserId),
    CONSTRAINT UserProfile_AK1 UNIQUE ( -- Composite ALTERNATE KEY.
        FirstName,
        LastName,
        GenderCode,
        BirthDate
    ),
    CONSTRAINT UserProfile_AK2 UNIQUE (Username) -- Single-column ALTERNATE KEY.
);

CREATE TABLE Friendship ( -- Stands for an associative entity type.
    RequesterId     INT      NOT NULL,
    AddresseeId     INT      NOT NULL, -- Fixed with a well-delimited data type.
    CreatedDateTime DATETIME NOT NULL,
    --
    CONSTRAINT Friendship_PK            PRIMARY KEY (RequesterId, AddresseeId), -- Composite PRIMARY KEY.
    CONSTRAINT FriendshipToRequester_FK FOREIGN KEY (RequesterId)
        REFERENCES UserProfile (UserId),
    CONSTRAINT FriendshipToAddressee_FK FOREIGN KEY (AddresseeId)
        REFERENCES UserProfile (UserId),
    CONSTRAINT FriendsAreDistinct_CK    CHECK       (RequesterId <> AddresseeId)
);

--
CREATE TABLE MyStatus ( -- Denotes an independent entity type.
    StatusCode CHAR(1)  NOT NULL,
    Name       CHAR(30) NOT NULL,
    --
    CONSTRAINT MyStatus_PK PRIMARY KEY (StatusCode),
    CONSTRAINT MyStatus_AK UNIQUE      (Name) -- ALTERNATE KEY.
); 

CREATE TABLE FriendshipStatus ( -- Represents an associative entity type.
    RequesterId       INT      NOT NULL,
    AddresseeId       INT      NOT NULL,
    SpecifiedDateTime DATETIME NOT NULL,
    StatusCode        CHAR(1)  NOT NULL,
    SpecifierId       INT      NOT NULL,
    --
    CONSTRAINT FriendshipStatus_PK             PRIMARY KEY (RequesterId, AddresseeId, SpecifiedDateTime), -- Composite PRIMARY KEY.
    CONSTRAINT FriendshipStatusToFriendship_FK FOREIGN KEY (RequesterId, AddresseeId)
        REFERENCES Friendship  (RequesterId, AddresseeId), -- Composite FOREIGN KEY.
    CONSTRAINT FriendshipStatusToMyStatus_FK   FOREIGN KEY (StatusCode)
        REFERENCES MyStatus    (StatusCode),
    CONSTRAINT FriendshipStatusToSpecifier_FK  FOREIGN KEY (SpecifierId)
        REFERENCES UserProfile (UserId),
    CONSTRAINT FriendsAreDifferent_CK          CHECK       (RequesterId <> AddresseeId)      
);

SELECT FS.StatusCode AS CurrentStatusCode
  FROM FriendshipStatus FS
 WHERE FS.RequesterId = 1750 --(a)
   AND FS.AddresseeId = 1748 --(b)
   AND FS.SpecifiedDateTime = (
                                  SELECT MAX(NestedFS.SpecifiedDateTime)
                                        FROM FriendshipStatus NestedFS
                                       WHERE NestedFS.RequesterId = FS.RequesterId
                                         AND NestedFS.AddresseeId = FS.AddresseeId
                              );
-- (a), (b) Those “fixed” values can of course be replaced by parameters.

SELECT FS.RequesterId,
       FS.AddresseeId,
       FS.SpecifiedDateTime,       
       FS.StatusCode AS CurrentStatusCode,
       FS.SpecifierId
  FROM FriendshipStatus FS
 WHERE FS.RequesterId = 1750
   AND FS.AddresseeId = 1748
   AND FS.SpecifiedDateTime = (
                                  SELECT MAX(NestedFS.SpecifiedDateTime)
                                        FROM FriendshipStatus NestedFS
                                       WHERE NestedFS.RequesterId = FS.RequesterId
                                         AND NestedFS.AddresseeId = FS.AddresseeId
                              );

CONSTRAINT FriendsAreDistinct_CK CHECK (RequesterId <> AddresseeId);