specified event occurs. It is a named PL/SQL unit that is stored in the database and can be invoked repeatedly. Unlike a stored procedure, you can enable and disable a trigger, but you cannot explicitly invoke it. While a trigger is enabled, the database automatically invokes it—that is, the trigger fires—whenever its triggering event occurs. While a trigger is disabled, it does not fire. You create a trigger with the CREATE TRIGGER statement. You specify the triggering event in terms of triggering statements, and the item they act on. The trigger is said to be created on or defined on the item—which is either a table, a view, a schema, or the database. You also specify the timing point, which determines whether the trigger fires before or after the triggering statement runs and whether it fires for each row that the triggering statement affects. If the trigger is created on a table or view, then the triggering event is composed of DML statements, and the trigger is called a DML trigger. If the trigger is created on a schema or the database, then the triggering event is composed of either DDL or database operation statements, and the trigger is called a system trigger. An INSTEAD OF trigger is either: A DML trigger created on a view or a system trigger defined on a CREATE statement. The database fires the INSTEAD OF trigger instead of running the triggering statement. Purpose of triggers Triggers can be written for the following purposes: Generating some derived column values automatically Enforcing referential integrity Event logging and storing information on table access Auditing Synchronous replication of tables Imposing security authorizations Preventing invalid transactions Data types The major datatypes in PL/SQL include NUMBER, CHAR, VARCHAR2, DATE and TIMESTAMP. Numeric variables variable_name number([P, S]) := 0; To define a numeric variable, the programmer appends the variable type NUMBER to the name definition. To specify the (optional) precision (P) and the (optional) scale (S), one can further append these in round brackets, separated by a comma. ("Precision" in this context refers to the number of digits the variable can hold, and "scale" refers to the number of digits that can follow the decimal point.) A selection of other data-types for numeric variables would include: binary_float, binary_double, dec, decimal, double precision, float, integer, int, numeric, real, small-int, binary_integer. Character variables variable_name varchar2(20) := 'Text'; -- e.g.: address varchar2(20) := 'lake