• The referencing-variables can be: • referencing old row/ table as var • referencing new row/ table as var • old vs new variable: • Only refer to new data for insertion events. • Only refer to old data old data for the deletion events. • Refer to both old and new data for update events. • row-level vs. statement-level: • The row-level trigger refers to both row and table variable and fires the trigger once for each row. • The statement-level trigger only refers to table variable and executes only once for each statement (event). If for each clause is omitted, the trigger is statement-level. • old | new table refers to a hypothetical read-only table before | after the event. • The condition is a predicate, same as the where clause. • The action corresponds to SQL statement.用中文详细讲解以上内容及知识点

以上内容是关于数据库中触发器的相关知识点。触发器是一种特殊的存储过程，它会在特定的数据库事件发生时自动执行，常用于记录日志、限制数据的插入、更新、删除等操作。以下是一些关于触发器的详细解释：

1. 引用变量可以是：引用旧行/表作为变量，引用新行/表作为变量。

2. 旧数据 vs 新数据变量：

对于插入事件，只引用新数据。

对于删除事件，只引用旧数据。

对于更新事件，引用旧数据和新数据。

3. 行级 vs 语句级：

行级触发器指的是对于每一行都会触发一次，同时可以引用行和表变量。

语句级触发器只触发一次，并只引用表变量。如果省略了“FOR EACH”子句，则触发器为语句级触发器。

4. 旧表 | 新表指的是一个虚拟的只读表，代表事件发生前或发生后的数据。

5. 条件是一个谓词，与 where 子句相同。

6. 操作对应于 SQL 语句，可以是 insert、update、delete 等操作。

相关问题

SyntaxError: The requested module /src/asstes/image/menu.png?import does not provide an export named menuUrl

This error message indicates that there is an issue with the syntax of the code or with the module import. The module specified in the code is not providing an export named "menuUrl", which the code is expecting.

To resolve this error, you can check the module code to ensure that it is exporting the correct variables or functions. You can also check the code that is importing the module to make sure it is referencing the correct export. Additionally, check for any typos or syntax errors in the code.

Python variables are references to obiects.

As an AI language model, I cannot have personal opinions. However, it is true that in Python, variables are references to objects. When a variable is assigned a value, it is actually pointing to a location in memory where the object is stored. Therefore, when a variable is used in an operation, it is actually referencing the object it was assigned to, rather than a copy of the object. This allows for efficient memory usage and flexible programming.