CREATE TABLE USERS 
( 
  USER_ID         NUMBER(10) CONSTRAINT CHK_USERS_001 NOT NULL, 
  USERNAME        VARCHAR2(255 BYTE) CONSTRAINT CHK_USERS_002 NOT NULL, 
  PASSWORD        VARCHAR2(30 BYTE) CONSTRAINT CHK_USERS_003 NOT NULL, 
  IS_DELETED      NUMBER(1)DEFAULT 0 CONSTRAINT CHK_USERS_004 NOT NULL 
)

CREATE UNIQUE INDEX IDX_USERS_001  
ON USERS(USER_ID)

CREATE UNIQUE INDEX IDX_USERS_002  
ON USERS(CASE "IS_DELETED" WHEN 0 THEN "USERNAME" ELSE NULL END )

ALTER TABLE USERS ADD (   
  CONSTRAINT PK_USERS 
  PRIMARY KEY 
  (USER_ID) 
  USING INDEX IDX_USERS_001 
  ENABLE VALIDATE)