另一個WHERE子句，如果沒有結果

Question

在mysql中的一個查詢中做到這一點的最佳方法是什麼？

SELECT * FROM products WHERE language = 1 

如果沒有結果

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

如果沒有結果

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

Answer 1

我已經編輯它使其工作，但它不再優雅。

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原來的（沒有最後一個也沒有）存在缺陷。

原來，這並不像我想象的那麼聰明。見下面的評論。如果第一個和第三個查詢返回數據，它將失敗。如果你只有一個聯盟，但不是兩個或更多聯盟，它確實有效。

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這是很容易在MySQL：

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) 

見FOUND_ROWS的討論（）和SQL_CALC_FOUND_ROWS這裏： http://dev.mysql.com/doc/refman/5.0/en/information-functions.html#function_found-rows

Answer 2

的MySQL缺乏的功能，以使這一個去
例子：DENSE_RANK窗函數，或TOP..WITH TIES

因此，而不使用臨時表或IF語句（無論如何需要MySQL中的存儲過程），然後在客戶端上將其過濾爲最低優先級值。也就是說，消耗行直到數值發生變化

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; 

即使不理想，這也會消除往返於MySQL服務器的往返行程。它也避免了重新評估EXISTS子句中的先前SELECTs

Answer 3

您可以使用類似以下內容：（編輯使用exists並根據評論限制）。

(
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) 
) 

您使用嵌套查詢和計數（*）檢查以前的查詢是否爲NULL。

Answer 4

嘗試：

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

似乎有沒有什麼好辦法，如果可能的話，你最好把它變成一個過程，無論是填充臨時表或返回RowSet對象的客戶端

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

如何永遠如果沒有可能，你似乎有限制二級和三級查詢：

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)) 

但這只是醜陋和緩慢。

也許與IS NULL左加入會比不存在，雖然