Question

当提现表没有数据[Account No: 100.200.330.444]且存款表没有数据[Account No: 100.200.330.444]时，则仅显示NULL。但我想显示0。

我创建虚拟表并制作main balance [Account No: 100.200.330.444]
我的代码，例如deposit_amount - withdraw_amount = Result
撤回表为空时，然后反馈null
我想显示main_balance = 0

CREATE VIEW xyz as select account_no as account_no,
 CASE  
  when sum(deposit_amount)=null then 0
  when sum(deposit_amount)=0 then 0
ELSE sum(deposit_amount)
END
-
 (select  
  CASE  
   when sum(withdraw_amount)=null then 0
   when sum(withdraw_amount)=0 then 0
  ELSE sum(withdraw_amount)
 END
 from withdraws
  )as balance from deposits group by account_no

Answer 1

我认为您应该以{{1}}的形式编写此查询：

JOIN

请注意，我假设每个帐户至少都有一笔存款（用于开设该帐户）。否则，您将需要模拟FULL JOIN（有关方法，请参见this question）。

Answer 2

不能在Null字段上使用算术运算符。空为Unknown，因此x = unknown始终是未知的。

如果要检查为空，则可以使用

IsNull: i.e. IsNull(Field)
is null: i.e. Where Field is null;
coalesce: i.e. coalesce(Field, valueIfNull)

Answer 3

由于MySQL对视图的限制，几乎不可能在MySQL视图中准确表达。如果要使用所有帐户，请从帐户表开始，并在select中使用相关子查询：

select a.account_no,
       ((select coalesce(sum(d.deposit_amount), 0)
         from deposits d
         where d.account_no = a.account_no
        ) -
        (select coalesce(sum(d.withdraw_amount), 0)
         from withdraws w
         where w.account_no = a.account_no
        )
       ) as balance
from accounts a;

MySQL不允许FROM子句中的子查询，这使其他表达方式更具挑战性。

在SQL中反馈NULL值

3 个答案: