排他って何だろう

DROP TABLE VBMArea CASCADE CONSTRAINTS;

CREATE TABLE VBMArea (
   AreaCode       CHAR(2)      NOT NULL,
   AreaName       VARCHAR2(10) NULL,
   UpdateTime     DATE         NOT NULL
);

CREATE OR REPLACE TRIGGER VBMArea_BIU BEFORE INSERT OR UPDATE ON VBMArea
REFERENCING OLD AS OLD NEW AS NEW FOR EACH ROW
DECLARE
BEGIN
   :new.UpdateTime := sysdate;
END;
/

ALTER TABLE VBMArea
       ADD  ( CONSTRAINT XPK_VBMArea PRIMARY KEY (AreaCode)
       USING INDEX);

strSQL = "SELECT * FROM VBMArea WHERE AreaCode='" & _
          txtAreaCode & "' FOR UPDATE NOWAIT"

Private Sub cmdUpdate_Click()
  Dim strErrText      As String
  Dim objDs           As Object
  Dim strSQL          As String
  Dim strUpdate       As String

  On Error GoTo errClick:

  MousePointer = vbHourglass
  strSQL = "SELECT * FROM VBMArea WHERE AreaCode='" & txtAreaCode & "' FOR UPDATE"
  Set objDs = pobjDb.DBCreateDynaset(strSQL, 0&)
  If objDs.EOF Then
      objDs.DbAddNew
  Else
    strUpdate = Format$(objDs("UpdateTime").Value, "YYMMDDHHNNSS")
    If mstrUpdate <> strUpdate Then
      If MsgBox("他のユーザにより更新されています(" & strUpdate & ")．" & _
       vbCrLf & "更新します", _
              vbOKCancel + vbExclamation, App.Title) = vbCancel Then
        GoTo exitClick:
      End If
    End If
    objDs.DbEdit
  End If
  objDs("AreaCode").Value = Trim$(txtAreaCode)
  objDs("AreaName").Value = Trim$(txtArea)
  objDs.DbUpdate

  cmdSearch.Enabled = True
  cmdUpdate.Enabled = False

exitClick:
  On Error Resume Next
  Set objDs = Nothing
  MousePointer = vbDefault
  Exit Sub

errClick:
  If pobjSess.LastServerErr = 0 Then
    If pobjDb.LastServerErr = 0 Then
      If Err <> 0 Then
        strErrText = Error$
      End If
    Else
      strErrText = pobjDb.LastServerErrText
      pobjDb.LastServerErrReset
    End If
  Else
    strErrText = pobjSess.LastServerErrText
    pobjSess.LastServerErrReset
  End If
  MsgBox strErrText, vbOKOnly + vbExclamation, App.Title
  Resume exitClick:
End Sub