如何对两个SUMmed SQL结果字段求和

问题描述

我有一个简单的表结构，其中包含 reagent_bottles 和 reagent_aliquots 的记录。

从瓶子创建等分试样，等分试样记录将外键bottle_id指向了瓶子id。

要获取特定试剂的总体积（同时计算瓶中的体积和等分试样中的体积），下面的查询可以部分完成此工作：

SELECT  
      COALESCE(SUM(rb.volume),0) AS bVolume,COALESCE(SUM(ra.volume),0) AS aVolume
FROM reagent_bottles AS rb
LEFT JOIN 
      reagent_aliquotes AS ra
ON rb.id = ra.bottle_id
WHERE reagent_id = 408;

返回以上试剂编号（408）：

bVolume | aVolume
       2|       6

添加bVolume和aVolume是正确的答案，即8。

问题是，如何重写上面的查询以创建一个新字段 total_volume ，将查询中的bVolume和aVolume相加？

我尝试过：

SELECT  
      COALESCE(SUM(rb.volume),0) AS aVolume,SUM(bVolume + aVolume) AS total_volume
FROM 
      reagent_bottles AS rb
LEFT JOIN 
      reagent_aliquotes AS ra
ON rb.id = ra.bottle_id
WHERE reagent_id = 408;

但收到错误未知列，字段列表中的bVolume ..！

解决方法

汇总原始列：

SELECT  
      COALESCE(SUM(rb.volume),0) AS bVolume,COALESCE(SUM(ra.volume),0) AS aVolume,SUM(COALESCE(rb.volume,0) + COALESCE(ra.volume,0)) AS total_volume
FROM 
      reagent_bottles AS rb
LEFT JOIN 
      reagent_aliquotes AS ra
ON rb.id = ra.bottle_id
WHERE reagent_id = 408;