在选择中的计算中使用连接计数答案

【问题标题】：use join count in calculation in select在选择中的计算中使用连接计数
【发布时间】：2014-10-28 07:06:19
【问题描述】：

SELECT e.qty * e.unit_cost * e.unit_no * (e.factor/100) AS Ext_price, c.cost_type, c.cost_dt, c.internal_cost_amt, c.client_cost_amt
FROM equipment e
LEFT JOIN CostAllocation c ON (c.object_row_id = e.row_id)

我现在想将Ext_price 除以与设备行匹配的CostAllocation 行数。我该怎么做？

【问题讨论】：

你能用这个select返回COUNT(c.object_row_id)并在客户端上做吗？

标签： mysql sql join count

【解决方案1】：

SELECT e.qty*e.unit_cost*e.unit_no*e.factor/(100*x.cnt) AS Ext_price,
       c.cost_type,
       c.cost_dt,
       c.internal_cost_amt,
       c.client_cost_amt
FROM equipment e
LEFT JOIN CostAllocation c ON c.object_row_id = e.row_id
LEFT JOIN
  (SELECT row_id,
          count(*) cnt
   FROM CostAllocation
   GROUP BY row_id) x ON c.object_row_id = x.row_id

【讨论】：

【解决方案2】：

试试下面：

select  g.*,g.Ext_price/(select count(*) from CostAllocation k where k.object_row_id=g.row_id) as youranswer
from 
(
SELECT e.row_id,e.qty*e.unit_cost*e.unit_no*e.factor/100 AS Ext_price, c.cost_type, c.cost_dt, c.internal_cost_amt, c.client_cost_amt
FROM equipment e
LEFT JOIN CostAllocation c ON c.object_row_id = e.row_id)g

【讨论】：

【解决方案3】：

由于 mysql 不支持分析功能，因此添加子查询（未经测试）可能是最简单的：

SELECT Ext_price, cost_type, cost_dt, internal_cost_amt
     , client_cost_amt
     , Ext_price / ( nullif( (select count(1) 
                              from CostAllocation c2
                              where c2.object_row_id = e.row_id), 0) )
FROM (
    SELECT e.qty*e.unit_cost*e.unit_no*e.factor/100 AS Ext_price
         , c.cost_type, c.cost_dt, c.internal_cost_amt, c.client_cost_amt
    FROM equipment e
    LEFT JOIN CostAllocation c 
        ON c.object_row_id = e.row_id
) as T

我添加了 nullif 以避免被零除。

【讨论】：