在一列上使用CASE語句並計算另一列的聚合

Question

我想在一列上使用case語句，並在條件滿足時計算另一列的聚合。

，我查詢看上去像這樣的樣本數據表（用於酒店爲例）：

| date | customer_name | customer_id | expense_type | billed_amount | 
| 08-02 | John Doe |  1  | personal |  120.00 | 
| 08-03 | John Doe |  1  | personal |  80.00 | 
| 08-04 | John Doe |  1  | corporate |  205.00 | 
| 08-02 | Adam Smith |  2  | corporate |  400.00 | 
| 08-03 | Adam Smith |  2  | personal |  300.00 | 
| 08-06 | Adam Smith |  2  | corporate |  150.00 | 

下面是SQL查詢我寫道：

Select 
    customer_name 
    , customer_id 
    , case when expense_type = 'personal' then sum(billed_amount) else 0 end as personal_bill 
    , case when expense_type = 'corporate' then sum(billed_amount) else 0 end as corporate_bill 
From payments 
Where date > '08-01' 
Group by 1, 2 

錯誤消息我得到是：

Column "expense_type" must appear in the GROUP BY clause or be used in an aggregate function 

當我嘗試組3列（以及1和2），我收到此錯誤消息：

Aggregates not allowed in GROUP BY clause 

最後，下面示出所希望的結果表：

| customer name | customer_id | personal_bill | corporate_bill | 
| John Doe |  1  |  200.00 |  205.00  | 
| Adam Smith |  2  |  300.00 |  550.00  | 

一種解決方案，我能想到是創建限制在哪裏部分中的「expense_type」兩個不同的子查詢（即where expense_type ='personal'），然後在主查詢中查詢它們，但這是很多類似的代碼，只有一行的區別。你能幫我以有效的方式寫這個查詢嗎？謝謝！

Answer 1

使用filter：

select 
    customer_name, 
    customer_id, 
    sum(billed_amount) filter (where expense_type = 'personal') as personal_bill, 
    sum(billed_amount) filter (where expense_type = 'corporate') as corporate_bill 
from payments 
where date > '08-01' 
group by 1, 2 

customer_name | customer_id | personal_bill | corporate_bill 
---------------+-------------+---------------+---------------- 
Adam Smith |   2 |  300.00 |   550.00 
John Doe  |   1 |  200.00 |   205.00 
(2 rows)  

Answer 2

總和整個CASE表達：

select 
    customer_name, 
    customer_id, 
    sum(case when customer_type = 'personal' 
      then billed_amount else 0 end) as personal_bill 
    sum(case when customer_type = 'corporate' 
      then billed_amount else 0 end) as corporate_bill 
from payments 
where date > '08-01' 
group by 1, 2 

Answer 3

試試這個： -

Select MIN(customer_name) AS customer_name, customer_id, 
     SUM(case when expense_type = 'personal' then billed_amount::double precision else 0 end) as personal_bill, 
     SUM(case when expense_type = 'corporate' then billed_amount::double precision else 0 end) as corporate_bill 
From payments 
Where date > '08-01' 
Group by customer_id 
ORDER BY customer_id;