Question

在mysql的一个查询中执行此操作的最佳方法是什么？

SELECT * FROM products WHERE language = 1

如果没有结果

SELECT * FROM products WHERE language = 2 AND country = 1

如果没有结果

SELECT * FROM products WHERE language = 2 AND country = 2

Answer 1

我已对其进行了编辑以使其正常工作，但它不再优雅。

原件（没有最后且不存在）有缺陷。

事实证明这并不像我想象的那么聪明。见下面的评论。如果第一个和第三个查询返回数据，它将失败。如果你只有一个联盟，但不是两个或更多联盟，它确实有效。

在mysql中很容易：

select SQL_CALC_FOUND_ROWS * from products where language = 1
union
SELECT * FROM products WHERE language = 2 AND country = 1 and found_rows()=0
union
SELECT * FROM products WHERE language = 2 AND country = 2 and found_rows()=0 
AND not exists(select * from products where language = 1)

请在此处查看found_rows（）和SQL_CALC_FOUND_ROWS的讨论： http://dev.mysql.com/doc/refman/5.0/en/information-functions.html#function_found-rows

Answer 2

您可以使用以下内容:(根据评论编辑使用EXISTS和LIMIT）。

(
SELECT * FROM products WHERE language = 1
)
UNION
(
SELECT * FROM products WHERE language = 2 AND country = 1
AND NOT EXISTS(SELECT count(*) FROM products WHERE language = 1 limit 1)
)
UNION
(
SELECT * FROM products WHERE language = 2 AND country = 2 
AND NOT EXISTS(SELECT count(*) FROM products WHERE language = 2 AND country = 1 limit 1)
AND NOT EXISTS(SELECT count(*) FROM products WHERE language = 1 limit 1)
)

使用嵌套查询检查并计算（*）先前的查询为NULL。

Answer 3

MySQL缺乏允许一次性使用的功能示例：DENSE_RANK窗口函数或TOP..WITH TIES

因此，不使用临时表或IF语句（无论如何都需要MySQL中的存储过程），然后在客户端上将其过滤为最低优先级值。也就是说，消耗行直到值改变

SELECT *, 1 AS priority FROM products WHERE language = 1
UNION ALL
SELECT *, 2 AS priority FROM products WHERE language = 2 AND country = 1
UNION ALL
SELECT *, 3 AS priority FROM products WHERE language = 2 AND country = 2
ORDER BY priority;

这确实会删除到MySQL服务器的往返，即使不理想。它还避免重新评估EXISTS子句中以前的SELECT

Answer 4

尝试：

select p.* 
from (select min(language) minlang, min(country) minctry 
      from products where language = 1 or
                          (language = 2 and country in (1,2)) ) c
join products p 
on p.language = c.minlang and (c.minlang=1 or p.country=c.minctry)

Answer 5

似乎没有好方法，如果可能的话，你最好把它变成一个过程，要么填充临时表，要么将RowSet对象返回给客户端

但是，如果没有任何可能，您似乎限制了二级和三级查询：

SELECT * FROM products WHERE language = 1

UNION ALL

SELECT * FROM products WHERE language = 2 AND country = 1 
   AND NOT EXISTS (SELECT * FROM products WHERE language = 1)

UNION ALL

SELECT * FROM products WHERE language = 2 AND country = 2
   AND NOT EXISTS (SELECT * FROM products WHERE language = 1 OR (language = 2 AND country = 1 ))

但那必须是丑陋和缓慢的。

使用IS NULL的LEFT JOIN可能比NOT EXISTS更好

如果没有结果，则另一个WHERE子句

5 个答案: