mysql 优化语句

优化前：

select b.type, ifnull(count(0),0) as qty from b_sell_quotationd a,b_sell_quotation b 
where b.quotationNo=a.quotationNo and a.status='N' and b.companycode='XXXXX' AND b.type='general';

优化后：

select  COUNT(0) from b_sell_quotationd a LEFT JOIN (SELECT * FROM b_sell_quotation WHERE type='general') b ON b.quotationNo=a.quotationNo WHERE b.companycode='XXXXXX' and a.status='N'