rows - 如何提高查询的性能

Question

select serialno, companyno, customer, sourcecode, itemclass, group, subgroup, 
       case when employeecd = 'A' then 1 else 0 end, 
       case when employeecd = 'B' then 1 else 0 end, 
       case when employeecd = 'C' then 1 else 0 end, 
from sourcemarket 
where serailno in select min(serialno) 
from sourcemarket 
join customer 
  on sourcemarket.sourcecode = customer.companyno 
 and sourcemarket.customer = customer.customer 
 and sourcemarket.sourcecode = customer.sourcecode 
group by Group by companyno,customer,sourcecode,itemclass,group,subgroup

在这里，我们正在为 CSNR/SRCD/ITCL/GRP/SGRP 的最小组合构建第一个客户和 EMP 值（A、B、C、... I、X）的文件