mysql - 如何（MySQL）乘以列然后求和行？

Question

我有这张桌子：

id    |   payment_id   | quantity  |  unit cost
1           633             1            750  
2           633             1            750 
3           632             2            750 
4           632             2            750 

我需要的是：

payemnt_id  |   total
    633           1500
    632           3000 

您也可以将其视为国家，然后您试图找到每个国家的总数。我确信有一些这样的教程，但 STFW 找不到。

Answer 1

您可以简单地将公式（表达式）放入SUM：

SELECT payment_id, SUM(`unit cost` * quantity) AS total
FROM myTable
GROUP BY payment_id

Answer 2

SELECT 
    payment_id, 
    SUM(subtotal) AS total
 FROM(
    SELECT
        payment_id,
        (unit_cost * quantity) AS subtotal
) AS t1
GROUP BY 
payment_id

Answer 3

SELECT payemnt_id, SUM(`unit cost` * quantity) as total
FROM Table1
GROUP BY payemnt_id

Answer 4

SELECT COALESCE (SUM(cost_field * quantity_field),0) AS tot FROM Table_Name    

我觉得这个不错，为什么呢？因为如果他们没有结果，它会给你零