如何提升这条SQL语句的速度？？？
SELECT zy_detail_charge.ward_code,zy_bill_item.code,sum(zy_detail_charge.charge_fee)
FROM 
zd_charge_item ,zy_detail_charge ,zy_bill_item,
(select distinct inpatient_no,admiss_times,ledger_sn from zy_receipt 
where account_date >='2007-01-01' and account_date < '2007-02-01' ) a
WHERE ( zy_detail_charge.charge_code = zd_charge_item.code ) and  
( zd_charge_item.bill_item_code = zy_bill_item.code ) and  
( zy_detail_charge.inpatient_no = a.inpatient_no ) AND  
( zy_detail_charge.admiss_times = a.admiss_times ) AND  
( zy_detail_charge.ledger_sn = a.ledger_sn ) AND
  zy_detail_charge.charge_status = '3' and 
 zy_detail_charge.ward_code = '3020100'
GROUP BY zy_detail_charge.ward_code,
  zy_bill_item.code;


zy_detail_charge 这个表的数据超过100万条记录，花了十多分钟才能出结果，请问如何优化？每个表都有索引

------解决方案--------------------

SQL code

--account_date时间列创聚集索引,zy_receipt表inpatient_no,admiss_times,ledger_sn建组合索引
--zy_detail_charge表inpatient_no,admiss_times,ledger_sn建组合索引，charge_code另建一索引
--zd_charge_item表Code建索引,zy_bill_item表建bill_item_code索引
SELECT  
    b.ward_code,
    c.code,
    sum(b.charge_fee) 
FROM  
    zd_charge_item  a
join
    zy_detail_charge b on b.charge_code=a.Code
join
    zy_bill_item c on a.bill_item_code=c.Code
where
    exists(select 1 from zy_receipt where inpatient_no=b.inpatient_no and admiss_times=b.admiss_times and ledger_sn= b.ledger_sn
    and account_date between '2007-01-01 ' and '2007-01-31 23:59:59') and b.charge_status=3 and b.ward_code='3020100'
group by     
    b.ward_code,c.code

------解决方案--------------------
使用存在谓词exists的效率比连接的效率高很多，它使用的内存小。