mysql初选条件

Question

如果主条件匹配，返回结果基于主条件，无论二级条件是否匹配，如果没有匹配结果，则返回二级条件的数据库。

以交换外汇为例，我有一个汇率表，我需要拿到美元汇率：英镑，如果没有，那么得到英镑汇率：美元。如果两者都有，只返还美元：英镑。

id origin_currency  target_currency  rate_from  rate_to 
1  1     2     1    0.75  
2  2     1     1    1.34 

对于上面的数据，查询sql应该只返回第一条记录。

我可以用下面的SQL来得到我想要的：

select * from exchange_rate 
where origin_currency = 1 and target_currency = 2 
union 
select * from exchange_rate 
where origin_currency = 2 and target_currency = 1 
limit 1 

然而，还有没有其他更好的解决办法？

Answer 1

你可以得到所有相关的行，由origin_currency = ? DESC和限制对它们进行排序的行数（或只是阅读你的结果的第一行）。请参阅SQL Fiddle。

SELECT 
    * 
FROM 
    currency 
WHERE 
    (origin_currency = 1 AND target_currency = 2) OR 
    (origin_currency = 2 AND target_currency = 1) 
ORDER BY 
    origin_currency = 1 DESC 
LIMIT 
    1 

Answer 2

尝试使用这个小提琴sql_fiddle

SELECT * 
FROM currency 
WHERE 
    (origin_currency, target_currency) = 
    (SELECT origin_currency, target_currency FROM currency 
    WHERE (origin_currency, target_currency)=(1,2) 
      UNION ALL 
      SELECT origin_currency, target_currency 
      FROM currency WHERE (origin_currency, target_currency)=(2,1) 
      LIMIT 1)