如何通过声明

Question

我写MySQL中的过程将通过DESC一个字符串值，从而，

IN `para` varchar(4) 
BEGIN 
    #Routine body goes here... 

if (para = 'asc') THEN 

SELECT officeCode, city, country 
from offices 
ORDER BY country ; 

ELSE 

SELECT officeCode, city, country 
from offices 
ORDER BY country DESC; 


END if; 
END 

该代码是好的，但我不知道是否有任何其他方法，使短，像这样的（第为参数时调用程序）：

BEGIN 
SELECT officeCode, city, country 
from offices 
ORDER BY country *para* ; 

END 
--------but this is not work------- 

Answer 1

你可以很容易地动态地根据para使用自己选择的任何脚本语言生成查询的文本，如果在你的解决方案工作。

Answer 2

这里有一个窍门，你可以用CASE表达式中使用：

SELECT officeCode, 
     city, 
     country 
FROM offices 
ORDER BY CASE WHEN para = 'asc' THEN Country ELSE '1' END ASC, 
     CASE WHEN para != 'asc' THEN Country ELSE '1' END DESC; 

Answer 3

你有两个选择。一种是与order by拨弄：

SELECT officeCode, city, country 
FROM offices 
ORDER BY (CASE WHEN para = 'asc' THEN country END), 
     country DESC; -- you could use a `case` here but it is not necessary 

或者，你可以使用一个事先准备好的声明：

set @sql = ' 
SELECT officeCode, city, country 
FROM offices 
ORDER BY para [dir]'; 

set @sql = replace(@sql, '[dir]', (case when para = 'asc' then 'asc' else 'desc' end)); 

prepare q from @sql; 
execute q; 

动态SQL的好处是，你有一个希望查询将使用索引。