oracle语法计算日期间隔

SELECT
 to_number( to_date(to_char(b.FBIZDATE, 'yyyy-mm-dd'), 'yyyy-mm-dd')-to_date(to_char(a.FBIZDATE, 'yyyy-mm-dd'), 'yyyy-mm-dd' )  )
  day 
FROM
 CT_BB_BreedingBatchManage a
 INNER JOIN CT_D_InternalReceive b ON a.fid = b.CFBATCHNOID