在这里举例：
    客户-》项目  一对多
    项目-》合同  一对多

khdw  客户表    id khmc
xmxx  项目表    id khbh 
htxx   合同表     id xmbh htje

1.查询某个客户的合同额(很简单，直接根据客户编号关联查询)

select sum(htje) as htje from htxx where qdsj>= '20180101' and qdsj<= '20201231' and  xmbh in
(select id from crm_xmxx where khbh = '指定客户的ID')

2.查询所有客户的合同额，分组显示：(思路：先按照三级合同表左连接查询项目信息和客户信息，然后使用聚合函数查询)

select sum(htje) as htje,khbh from (
select a.htje,b.khbh,c.khmc from htxx a left join xmxx b on a.xmbh = b.id left join khdw c on b.khbh= c.id)
group by khbh