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This article introduces the relevant knowledge of "what is a trigger". In the operation of actual cases, many people will encounter such a dilemma. Then let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

I. the concept of trigger

A trigger is a special type of stored procedure that is different from the one we introduced earlier. Triggers are mainly triggered by events, while stored procedures can be called directly by the name of the stored procedure. When you perform operations such as UPDATE, INSERT, or DELETE on a table, SQL Server automatically executes the SQL statements defined by the trigger, ensuring that the data must be processed in accordance with the rules defined by these SQL statements.

Second, the function of trigger

The main function of triggers is to achieve complex referential integrity and data consistency that cannot be guaranteed by primary keys and foreign keys. In addition, triggers have many other different functions:

A. strengthen the constraint

Triggers can implement more complex constraints than CHECK statements.

B. track changes

Triggers detect operations within the database so as not to allow unauthorized specified updates and changes in the database.

C. cascade operation

Triggers can detect operations in the database and automatically cascade the contents of the entire database. For example, a trigger on a table contains data operations (such as delete, update, insert) on another table, which in turn causes the trigger on that table to be triggered.

D. calls to stored procedures

In response to database updates, triggers can call one or more stored procedures and even operate outside the DBMS itself through calls from external procedures.

Thus it can be seen that triggers can solve some problems such as advanced forms of business rules or complex behavior restrictions and the implementation of custom records. For example, a trigger can find out the difference in the state of a table before and after data modification, and perform certain processing according to this difference. In addition, multiple triggers of the same type (INSERT, UPDATE, DELETE) of a table can handle the same data operation in many different ways.

Overall, the performance of triggers is usually low.

III. Types of triggers

SQL Server 2000 supports two types of triggers: AFTER triggers and INSTEAD OF triggers.

AFTER triggers require that triggers are triggered only after an operation (INSERT, UPDATE, DELETE) has been performed and can only be defined on the table. You can define multiple triggers for the same operation on the table.

An INSTEAD OF trigger means that it does not perform the operations it defines (INSERT, UPDATE, DELETE), but only executes the trigger itself. You can define either an INSTEAD OF trigger on a table or an INSTEAD OF trigger on a view, but only one INSTEAD OF trigger can be defined for the same operation.

This is the end of what is a trigger. Thank you for your reading. If you want to know more about the industry, you can follow the website, the editor will output more high-quality practical articles for you!

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