如何编写这个复杂的查询

Question

 Master Table
nID  Elementtype elementval 
1    volume         5          
1    rating         +          
1    volume         3          
1    rating         -          
2    volume         2          
2    rating         *            
2    volume         4         
2    rating         +          
3    volume         3          
3    rating         + 

---

client      table
  nID       client   
   1        halkins    
   2        narnia
   3        avatar
   4        narnia   
   5        newsco
   6        halkins 

如何获得客户端三种类型的体积平均值和总和，例如对于halkins

client     No(sum)       volumeavg      ngve   potve    neutral
 halkins    4                 4          1       1         0

如何在同一查询中基本上获得其他值volumeavg 或者是他们的任何其他方法体积是体积的平均值，对于halkins 5 + 3/2 = 4

Answer 1

您需要join表格，然后分组结果：

SELECT   client,
         COUNT(*) AS `No(sum)`,
         AVG(IF(Elementtype='volume', elementval, NULL)) AS volumeavg,
         SUM(Elementtype='rating' AND elementval='-') AS ngve,
         SUM(Elementtype='rating' AND elementval='+') AS potve,
         SUM(Elementtype='rating' AND elementval='*') AS neutral
FROM     Master JOIN client USING (nID)
GROUP BY nID

在sqlfiddle上查看。