--实现两张表的数据相加
drop table a;
create table a(
currency_type character varying,
c_val float
);
insert into a(currency_type,c_val) values('CNY',10);
drop table b;
create table b(
currency_type character varying,
c_val float
);
insert into b(currency_type,c_val) values('CNY',20);
insert into b(currency_type,c_val) values('GY',20);
insert into b(currency_type,c_val) values('CNY',30);
drop table c;
create table c(
currency_type character varying,
c_val float
);
insert into c(currency_type,c_val) values('USD',30);
多张表的不同字段的值对应的数据进行相加
select d.currency_type,sum(c_val)
from (select currency_type,c_val from a
union all
select currency_type,c_val from b
union all
select
