列值在表中作为列视图

Question

我也由此表：

CityID  ParamName  ParamValue 
------ -----------  ---------- 
    1  Temperature  23  
    1  Humidity   56 
    2  Temperature  27 
    2  Humidity   49 

我想创建一个视图，它想是这样的：

CityID Temperature  Humidity 
------ -----------  -------- 
    1   23    56 
    2   27    49 

我的数据库引擎是MySQL。

有人可以帮我定义这样的观点吗？任何帮助将不胜感激。

Answer 1

MySQL没有一个PIVOT功能，因此你需要给你CASE声明：

SELECT CityId, 
    SUM(CASE WHEN ParamName = 'Temperature' THEN ParamValue END) Temperature, 
    SUM(CASE WHEN ParamName = 'Humidity' THEN ParamValue END) Humidity 
FROM yourTable 
GROUP BY CityId 

见SQL Fiddle with Demo

Answer 2

像

select someTable.CityID,temps.ParamValue as Temperature,Hums.ParamValue as Humidity 
From SomeTable 
inner join someTable temps 
On someTable.CityID = temps.CityID and temps.ParamName = 'Temperature' 
inner join SomeTable hums 
On someTable.CityID = hums.CityID and hums.ParamName = 'Humidity'