PostgreSQL有條件求和

Question

我有一個transactions表，它包含一個類別（category_id），金額（amount）和一個標誌（managed），它們可以是真或假。

我想顯示所有類別的列表，其中包括託管和非託管交易總數，例如

Category | managed_transactions | unmanaged_transactions 
Cat 1 |  124000   |  54000 
Cat 2 |  4000    |  0 
Cat 3 |  854000   |  1000000 

有沒有辦法做這樣的事情

Select category_id, 
     sum(amount) if (managed is true) as managed_transactions, 
     sum(amount) if (managed is false) as unmanaged_transactions 
from transactions 

我顯然粘在if managed is true部分...

Answer 1

享受！

SELECT 
    category_id, 
    SUM(CASE WHEN managed THEN amount ELSE  0 END) AS managed_transactions, 
    SUM(CASE WHEN managed THEN  0 ELSE amount END) AS unmanaged_transactions 
FROM 
    transactions 
GROUP BY 
    category_id 
ORDER BY 
    category_id