在表中输入一行时，如何检查另一个表的约束? [英] How do you check constraints from another table when entering a row into a table?

问题描述

假设我有两个表 Account 和 Withdraw.帐户具有属性余额.每次在提款表中输入一个值时，我想检查是否有足够的余额来这样做.如果是，我想从余额中减去该金额.

这不是我想要做的，而是我需求的简化版本.

PS:检查约束"是正确的短语吗?我不太确定.谢谢！

解决方案

这是一个简短的例子:

INSERT INTO mytable(身份证，姓名)选择 1，'测试'不存在的地方(选择 IDFROM myanother_table哪里 id = 1)

<块引用>

注意:如果不理解或不看，很难假设任何事情OP的代码.所以我只是提供了例子.

Say I have two tables Account and Withdraw. Account has an attribute Balance. Every time a value is entered into the table Withdraw I would like to check if there is sufficient balance to do so. If yes, I'd like to subtract that amount from the Balance.

This isn't exactly what I want to do, but a simplified version of my requirement.

PS: Is "check constraints" the correct phrase? I'm not really sure. Thanks!

解决方案

This is short example:

INSERT INTO mytable 
            (id, name) 
SELECT 1, 'test' 
WHERE  NOT EXISTS(SELECT id 
                  FROM   myanother_table 
                  WHERE  id = 1) 

Note: Its hard to assume anything without understand or taking a look of OP's code. So i have just provided example.

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