查找SQL约束：SELECT COUNT（*）from tBoss< 2

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问题描述

我想限制表中的条目。让我们说在 tBoss 表中。是否有SQL约束检查表中当前有多少个元组？喜欢

I'd like to limit the entries in a table. Let's say in table tBoss. Is there a SQL constraint that checks how many tuples are currently in the table? Like

SELECT COUNT(*) from tBoss < 2

Firebird说：

Firebird says:

Norbert

推荐答案

您的数据库有触发器吗？如果是，添加一个触发器，回滚将添加超过2行的任何插入...

Does your database have triggers? If so, Add a trigger that rolls back any insert that would add more than 2 rows...

Create Trigger MyTrigName
For Insert On tBoss
As
    If (Select Count(*) From tBoss) > 2
       RollBack Transaction

但是为了直接回答你的问题，你想要的谓词将select子查询放在括号中。像这样...

but to answer your question directly, the predicate you want is to just put the select subquery inside parentheses. like this ...

  [First part of sql statement ]
  Where (SELECT COUNT(*) from tBoss) < 2

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