將總共兩個SUM函數

Question

我該怎麼做來總結這樣的結果的查詢使用PostgreSQL？：

SELECT SUM(value) FROM credit_balance 

SELECT SUM(value) FROM debit_balance 

我都試過，但它不工作：

SELECT SUM(SELECT SUM(value) FROM credit_balance UNION ALL SELECT SUM(value) FROM debit_balance) 

Answer 1

SELECT 
    (SELECT SUM(value) FROM credit_balance) + (SELECT SUM(value) FROM debit_balance) 

Answer 2

您在sum之後缺少字段名稱。你可以寫使用聯合查詢所有如下：

select sum(val_sum) from (SELECT SUM(value) as val_sum FROM credit_balance UNION ALL SELECT SUM(value) as val_sum FROM debit_balance) as united_table; 

OR

select sum(value) from (SELECT value FROM credit_balance UNION ALL SELECT value FROM debit_balance) as united_table.