如果没有结果，则另一个 WHERE 子句

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

我已经对其进行了编辑以使其工作，但它不再优雅。

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原件（没有最后一个且不存在）有缺陷。

事实证明这并不像我想象的那么聪明。请参见下面的 cmets。如果第一个和第三个查询返回数据，它将失败。如果您只有一个工会，而不是两个或更多工会，它确实有效。

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在mysql中很简单：

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)

在此处查看有关 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

您可以使用类似以下的内容：（编辑为使用基于 cmets 的 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)
)

您使用嵌套查询和 count(*) 检查以前的查询是否为 NULL。

Answer 3

好像没有什么好办法，如果可能的话，你最好把它做成一个过程，要么填充临时表，要么将RowSet对象返回给客户端

• SELECT-ing data from stored procedures
• MySql stored procedures: How to select from procedure table?

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 ))

但这一定是丑陋而缓慢的。

也许 LEFT JOIN with IS NULL 会比 NOT EXISTS 好

Answer 4

试试：

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

MySQL 缺少一次性实现此功能的功能
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），然后在客户端将其过滤为最低优先级值。即消费行直到值改变

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;

这确实消除了到 MySQL 服务器的往返，即使不是很理想。而且它还避免了在 EXISTS 子句中重新评估以前的 SELECT