Ultimately, triggers allow TSQL to be executed when write operations occur, regardless of the source of those writes. Each can be valuable depending on the circumstances and the type of code that needs to be managed via triggers. In these scenarios, the INSERT, UPDATE, or DELETE operation never occurs, and the contents of the trigger are executed instead. INSTEAD OF triggers replace the calling write operation. If the trigger fails, the original operation also fails. AFTER triggers take place after data is written to the table and are a distinct and separate set of operations that execute as part of the same transaction that wrote to the table, but after that write occurs. Triggers may be defined as INSTEAD OF or AFTER the write operation. MERGE operations fire triggers based on the operations performed by each operation within the MERGE statement. Triggers may be defined to execute on INSERT, UPDATE, DELETE, or any combination of those operations. In contrast, DDL (data definition language) triggers on the database or server are typically more focused and less harmful to performance.Ī trigger is a set of code that is evaluated when a data change is made to a table. This article focuses on DML triggers on tables as they tend to bear the brunt of over-usage. SQL Server triggers can be defined on the server, database, or table and allow code to automatically execute when specific actions occur. The challenges posed by triggers are also seen in MySQL, PostgreSQL, MongoDB, and many others. While all demos in this article are in SQL Server, the advice presented here is universal across database platforms. This article briefly reviews DML (data manipulation language) triggers and delves into a discussion about how triggers can be efficiently and effectively used as well as when triggers can dig developers into holes that are hard to escape. They can provide immense utility at a low cost, but are often misused by developers, DBAs, and architects alike, resulting in performance bottlenecks or maintainability challenges. SQL Server triggers are a subject of both functionality and controversy on any database platform.
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