Mysql使用连接从两个表中选择总和

Question

我想从两个表政策和付款中获取余额值。 MySQL 代码如下：

SELECT Sum(policy.premium)                                                   AS 
       `total` 
       , 
       (SELECT Sum(payments.amount) 
        FROM   payments 
        WHERE  ( payments.date_paid BETWEEN '2019-03-01' AND '2019-03-31' )) AS 
       `paid` 
FROM   `policy` 
       LEFT JOIN payments 
              ON policy.code = payments.code 
WHERE  ( policy.st BETWEEN '2019-03-01' AND '2019-03-31' ) 
       AND policy.trn_type = 0 

paid 列返回给 null，我怎样才能得到总和已付之间的差额。

Answer 1

您可以像这样计算 2 个值：

select t.total, coalesce(t.paid, 0) paid, (t.total - coalesce(t.paid, 0)) diff from (
  select 
    (select sum(premium) from policy where st between '2019-03-01' and '2019-03-31' and policy.trn_type=0) total,
    (select sum(amount) from payments where date_paid between '2019-03-01' and '2019-03-31') paid
) t

Answer 2

付费栏目就是这部分：

SELECT SUM(payments.amount) 
FROM payments 
WHERE payments.date_paid BETWEEN '2019-03-01' AND '2019-03-31'

如果返回 null，请先检查数据中的内容