在多个案例条件下联接2个表

Question

我在Postgres数据库上使用pgAdmin。我正在尝试获得以下结果（金额是随机的）：

为此，我需要查询以下两个表：帐户和交易

我不确定如何将总和（金额）结果分成1列。我尝试了以下方法：

select SUM(
    CASE WHEN debit_account_id = 1 then amount
        when credit_account_id = 1 then amount * (-1) else 0 end),
        SUM(
    CASE WHEN debit_account_id = 2 then amount
        when credit_account_id = 2 then amount * (-1) else 0 end)
    from transactions
    where entity_id = 1

，依此类推，直到account_id6。这将为我提供每个帐户的正确金额，但每个结果都在新列中。如何将其结合起来，使结果看起来像上面的示例？

Answer 1

您可以使用UNION ALL。

select debit_account_id account_id, -amount from transactions
union all 
select credit_account_id account_id, amount from transactions;

现在您将数据放在一栏中

Answer 2

我将在不同的查询中将每个帐户的借方和贷方加起来，然后将它们加入帐户表中：

SELECT account_name, sum_credut - sum_debit AS balance
FROM   accounts a
JOIN   (SELECT   credit_account_id, SUM(amount)
        FROM     transfer
        GROUP BY credit_account_id) c ON a.id = c.credit_account_id
JOIN  (SELECT    debit_account_id, SUM(amount)
        FROM     transfer
        GROUP BY debit_account_id) d ON a.id = d.debit_account_id

Answer 3

我为此建议一个横向连接：

select a.account_name,
       sum(v.signed_amount) as total_amount
from transactions t left join lateral
     (values (t.debit_account_id, t.amount), 
             (t.credit_account_id, - t.amount)
     ) v(account_id, signed_amount) join
     account a
     on a.id = v.account_id
group by a.account_name;

我在任何表格中都没有看到entity_id，所以我不知道那是哪里来的。