Define concept behind trigger. How statement level trigger differ from row level trigger? Explain.

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Trigger: A trigger is a stored procedure in a database that automatically invokes whenever a special event in the database occurs. For example, a trigger can be invoked when a row is inserted into a specified table or when certain table columns are being updated. OR,

 A trigger is a procedure that is automatically invoked by the DBMS in response to changes to the database and is specified by the database administrator (DBA). A database with a set of associated triggers is generally called an active database. These databases are very difficult to be maintained because of the complexity that arises in understanding the effect of these triggers. In such a database, DBMS initially verifies whether the particular trigger specified in the statement that modifies the database) is activated or not, prior to executing the statement. If the trigger is active then DBMS executes the condition part and then executes the action part only if the specified condition is evaluated to true. It is possible to activate more than one trigger within a single statement.

ROW Level Trigger

A row-level trigger is fired each time the table is affected by the triggering statement. For example, if an UPDATE statement updates multiple rows of a table, a row trigger is fired once for each row affected by the UPDATE statement. If a triggering statement affects no rows, a row trigger is not run. Row triggers are useful if the code in the trigger action depends on data provided by the triggering statement or rows that are affected. 

Statement level Trigger

A statement trigger is fired once on behalf of the triggering statement, regardless of the number of rows in the table that the triggering statement affects, even if no rows are affected. For example, if a DELETE statement deletes several rows from a table, a statement-level DELETE trigger is fired only once. Statement triggers are useful if the code in the trigger action does not depend on the data provided by the triggering statement or the rows affected. 



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