What are “triggers” in SQL?
Triggers are special procedures that are automatically executed when a table is inserted, updated, or deleted.
Triggers are generally used for check constraints which are more complex constraints. For example, when you perform an update, insert, or delete operation, the system will automatically invoke the corresponding trigger on the table.
SQLServer2005 triggers can be divided into two categories: DML triggers and DDL triggers, which DDL triggers they will affect a variety of Data Definition Language statements to stimulate the statement, these statements are create, alter, drop statement.
DML triggers are divided into:
1, after trigger (triggered after)
a, insert trigger
b, update trigger
c, delete trigger
2, beforeof trigger (triggered before)
which after trigger requires that only after the execution of an operation insert, update, delete trigger is triggered, and can only be defined on the table. An insteadof trigger does not perform the operation it defines (insert, update, delete) but only performs the trigger itself. Insteadof triggers can be defined both on the table and on the view.
The trigger has two special tables: the insert table (instered table) and the delete table (deleted table). These two are logical and virtual tables. There are two tables that the system creates in memory and does not store in the database. Moreover, both tables are read-only and can only read data but not modify it. The result of these two tables is always the same structure as the table to which the trigger is applied. When the trigger completes its work, the two tables are deleted; the Inserted table contains the inserted or modified data, and the Deleted table contains the updated or deleted data.
Updating the data means deleting a table record and then adding a record. The inserted and deleted tables will then have the updated data. Note that: the trigger itself is a transaction, so in the trigger can modify the data inside some special checks. If it does not meet the requirements, you can use the transaction rollback to undo the operation.
What sql triggers do
Used to enforce compliance with complex business rules or requirements. For example, you can control whether new orders are allowed to be inserted based on a customer’s current account status.
Triggers can also be used to enforce referential integrity so that when rows are added, updated, or deleted in multiple tables, the relationships defined between those tables are preserved. However, the best way to enforce referential integrity is to define primary and foreign key constraints in the relevant tables. If you use a database relationship diagram, you can create relationships between tables to automatically create foreign key constraints.
DELIMITER| CREATETRIGGER`<databaseName>`. `<triggerName>` <[BEFORE|AFTER]><[INSERT|UPDATE|DELETE]> ON<tableName> FOREACHROW BEGIN –dosomething END|
Triggers can cascade changes through related tables in the database; however, these changes can be enforced more efficiently by cascading referential integrity constraints. Triggers can enforce more complex constraints than those defined with CHECK constraints. Unlike CHECK constraints, triggers can reference columns in other tables. For example, a trigger can use a SELECT in another table to compare inserted or updated data, as well as to perform other operations such as modifying data or displaying user-defined error messages. Triggers can also evaluate the state of a table before and after data modifications and take countermeasures based on the differences. Multiple triggers of the same type (INSERT, UPDATE, or DELETE) on a table allow multiple different responses to the same modification statement.
Constraints and triggers each have their advantages in specific situations. The main benefit of triggers is that they can contain complex processing logic using Transact-SQL code. As a result, triggers can support all the functionality of constraints; however, it is not always the best approach for the given functionality. Entity integrity should always be enforced at the lowest level by indexes that are either part of the PRIMARYKEY and UNIQUE constraints or created independently of the constraints. Assuming that the functionality can satisfy the functional requirements of the application, domain integrity should be enforced through the CHECK constraint, while referential integrity (RI) should be enforced through the FOREIGNKEY constraint. Triggers are extremely useful when the functionality supported by the constraint does not meet the functional requirements of the application.
For example, unless the REFERENCES clause defines a cascading reference operation, the FOREIGNKEY constraint can only validate a column value with a value that exactly matches a value in another column.
The CHECK constraint can validate column values only against a logical expression or another column in the same table. If an application requires that a column value be validated against a column in another table, you must use a trigger. Constraints can only pass error messages through standard system error messages. Triggers must be used if the application requires (or can benefit from) customized messages and more complex error handling.
Triggers can cascade changes through related tables in the database; however, these changes can be enforced more efficiently by cascading referential integrity constraints. Triggers can disable or roll back changes that violate referential integrity, thereby canceling the attempted data modification. Such triggers may come into play when a foreign key is changed and the new value does not match the primary key. For example, an insert trigger could be created on titlesauthor.title_id to cause it to roll back an insert if the new value does not match one of the values in titles.title_id. However, it is common to use FOREIGNKEY for this purpose.
If constraints exist on the trigger table, they are checked after the INSTEADOF trigger executes but before the AFTER trigger executes. If the constraints are broken, the INSTEADOF trigger operation is rolled back and the AFTER trigger is not executed. Whether or not triggers can be created on views There is nothing in the SQLServer™ online book series that says triggers cannot be created on views, and the syntax explanations indicate that the ON of CREATETRIGGER can be followed by a view. However, this does not seem to be the case and many experts also say that triggers cannot be created on views. I also made a special test and it is true, no trigger can be created on it, no matter it is a normal view or index view, is it really true? Please click on the details, but there is no denying it: triggers are rejected when created on temporary or system tables.
What are sql triggers for and how do they work?
Triggers are used to ensure data consistency with. He is divided into two kinds of one is triggered before the operation of the other is triggered after the operation, mainly on the data insertion, deletion, modification and other actions to track and make the corresponding action!