两列连接到同一个表的同一列，怎么写sql查询语句？

Question

表product中有一列incurrency表示商品进货的货币种类，另一列outcurrency表示货品卖出的货币种类,都是货币的索引。 表currency中有货币的索引。 如果我想要直接把进货卖货的货币都检索出来，要怎么写sql查询语句呢？ 我原本是这样写的：

select currency.name as incurrencyname, currency.name as outcurrencyname
from product
inner join currency 
on product.in_currency = currency.id 
on product.out_currency = currency.id 

但是查询的时候会显示：

ERROR: syntax error at or near "on"

Answer 1

select currency_in.name as incurrencyname, currency_out.name as outcurrencyname
from product
inner join currency as currency_in
on product.in_currency = currency_in.id
inner join currency as currency_out
on product.out_currency = currency_out.id