sql语句问题:多表关联查询问题?在找不到条件时如何保留a表的group by...答:select a.bank_num , a.trans_date , a.mec_code , isNull(b.mec_desc,''), a.txn_sub_num ,isNull(c.trans_desc,''), a.sum_num , a.success_num from (select bank_num, substr(trans_date,1,6) as trans_date, mec_code, txn_sub_num, sum(sum_num) as sum_num, ...