如何利用數據庫建立Union 約束

     爲了少在Client做DB的邏輯判斷,可以儘量用DB的Union 約束來限制數據的重複輸入,
那麼怎麼建立Group_users表中GroupID,userId的Union 約束呢? 如下

DROP TABLE USER_GROUP CASCADE CONSTRAINTS ;

CREATE TABLE USER_GROUP (
  ID        NUMBER        NOT NULL,
  USER_ID   NUMBER (8)    DEFAULT 0 NOT NULL,
  GROUP_ID  NUMBER (8)    DEFAULT 0 NOT NULL,
  CONSTRAINT PK_USERCPHASE
  PRIMARY KEY ( ID )
    USING INDEX
     TABLESPACE NEWKM_IDX PCTFREE 10
     STORAGE ( INITIAL 65536 NEXT 1048576 PCTINCREASE 0 ))
   TABLESPACE NEWKM_DAT
   PCTFREE 10
   PCTUSED 40
   INITRANS 1
   MAXTRANS 255
  STORAGE (
   INITIAL 65536
   MINEXTENTS 1
   MAXEXTENTS 2147483645
   FREELISTS 1 FREELIST GROUPS 1 )
   NOCACHE;

ALTER TABLE USER_GROUP ADD  CONSTRAINT FK_USER_GRO_REFERENCE_GROUPS
 FOREIGN KEY (GROUP_ID)
  REFERENCES NEWKM.GROUPS (ID) ;

ALTER TABLE USER_GROUP ADD  CONSTRAINT FK_USER_GRO_REFERENCE_USERS
 FOREIGN KEY (USER_ID)
  REFERENCES NEWKM.USERS (ID) ;

CREATE UNIQUE INDEX IX_USERGROUP_1 ON
  "NEWKM".USER_GROUP(USER_ID, GROUP_ID)
  TABLESPACE NEWKM_IDX PCTFREE 10  STORAGE(INITIAL 65536 NEXT 1048576 PCTINCREASE 0 )
;

GRANT SELECT ON USER_GROUP TO NEWEPP;

COMMENT ON TABLE USER_GROUP IS '羣組人員檔(USER_GROUP)';

COMMENT ON COLUMN USER_GROUP.GROUP_ID IS '羣組名稱序號';
COMMENT ON COLUMN USER_GROUP.ID IS '序號';
COMMENT ON COLUMN USER_GROUP.USER_ID IS '人員序號';