Postgresql如何跨两个不同的表求和？

我有两张桌子：

|currency_type|user_id|amount|
|-------------|-------|------|
|          aaa|    404|  2000|
|          bbb|    404|   300|
|          ccc|     22|   444|

如何分别获得每种货币类型的两个表（金额+余额）的总和？用户id=404的结果示例 性能对我来说很重要，所以越快越好

|currency_type|user_id|    sum|
|-------------|-------|-------|
|          aaa|    404|   2030|
|          bbb|    404|    300|
|          xxx|    404|     50|

试试这个：

create table t1 (currency_type text,user_id int,amount int);
create table t2 (currency_type text,user_id int,balance int);

insert into t1 values
('aaa',    404,  2000),
('bbb',    404,   300),
('ccc',     22,   444);

insert into t2 values
('aaa', 404, 30),
('xxx', 404, 50);

with t1t2 as (
  select currency_type, user_id, amount from t1
  union all
  select currency_type, user_id, balance as amount from t2
)
select currency_type, user_id, sum(amount)
from t1t2
where user_id = 404
group by currency_type, user_id

结果

货币类型|用户id |总和 :------------ | ------: | ---: aaa | 404 | 2030 bbb | 404 | 300 xxx | 404 | 50 示例

解释

只需合并两个表，但在查询中暂时将余额重命名为amount。现在，余额和金额在同一张表中。剩下的就是每个货币类型/用户id组合的合计金额列

create table t1 (currency_type text,user_id int,amount int);
create table t2 (currency_type text,user_id int,balance int);

insert into t1 values
('aaa',    404,  2000),
('bbb',    404,   300),
('ccc',     22,   444);

insert into t2 values
('aaa', 404, 30),
('xxx', 404, 50);

with t1t2 as (
  select currency_type, user_id, amount from t1
  union all
  select currency_type, user_id, balance as amount from t2
)
select currency_type, user_id, sum(amount)
from t1t2
where user_id = 404
group by currency_type, user_id

currency_type | user_id | sum :------------ | ------: | ---: aaa | 404 | 2030 bbb | 404 | 300 xxx | 404 | 50