如何對兩個 SUMmed SQL 結果字段求和

Question

我有一個簡單的表結構，用於保存試劑瓶和試劑等分的記錄。

等分試樣是從瓶子創建的，等分試樣記錄有一個外鍵，bottle_id，到瓶子 id。

要獲得特定試劑的總體積（計算瓶子中的體積和等分試樣中的體積），以下查詢部分完成了這項工作：

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;

返回上述試劑 ID (408)：

bVolume | aVolume
       2|       6

bVolume 和 aVolume 相加，是正確答案，即 8。

問題是，如何重寫上述查詢以在查詢中創建新字段total_volume 、求和 bVolume 和 aVolume ？

我試過這個：

SELECT  
      COALESCE(SUM(rb.volume), 0) AS bVolume , 
      COALESCE(SUM(ra.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;

但是在字段列表中收到錯誤，未知列，bVolume ..！

Answer 1

對原始列求和：

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;