如何在不添加整列的情况下选择不同列的总和

Question

a.id  c_id  p_type  paid     date
1      6    FLOUR   100.00  2015-06-22
2      7    OIL     50.00   2015-06-21
3      6    FLOUR   242.00  2015-06-24
4      6    FLOUR   392.00  2015-06-26
5      7    OIL     200.00  2015-07-29
6      7    OIL     300.00  2015-07-25

我一直试图只从不同的表中选择值的总和，我试过

SELECT customer.first_name, customer.last_name, credit.quantity, 
       credit.date_supplied, credit.unit_price, credit.p_type,
       ROUND(SUM(account.amount_paid))
AS amount_paid, account.date_paid FROM credit, customer 
LEFT OUTER JOIN account 
ON account.c_id = customer.c_id ;

我想显示产品类型面粉和油的列总和，但我只得到表中总量的总和。我很感激你的帮助。

Answer 1

您可以按类型分组，然后在组中使用sum。

Answer 2

分组依据将对事物进行分组，然后对给定的组执行操作。因此，它将评估将其分为两组，即油和面粉，然后对每一组进行总和。

SELECT 
  customer.first_name
, customer.last_name
, credit.quantity
, credit.date_supplied
, credit.unit_price
, credit.p_type
, ROUND(SUM(account.amount_paid)) AS amount_paid
, account.date_paid 
FROM credit
LEFT JOIN customer on credit.??? = customer.??? 
LEFT JOIN account ON account.c_id = customer.c_id 
GROUP BY credit.p_type;