Triggers are used to define code that is executed when certain actions or events occur.
The Syntax for Creating a Database Trigger
CREATE [OR REPLACE] TRIGGER trigger_name
{BEFORE|AFTER} verb_list ON table_name
[[REFERENCING correlation_names] FOR EACH ROW [WHEN (condition)]]
DECLARE
declarations
BEGIN
pl/sql_code
END;
verb_list -- The SQL verbs that fire the trigger.
table_name -- The table on which the trigger is defined.
correlation_names -- Allows you to specify correlation names other than the default of OLD and NEW.
condition -- An optional condition placed on the execution of the trigger.
declarations -- Consists of any variable, record, or cursor declarations needed by this PL/SQL block.
SQL> CREATE TABLE to_table
2 (col1 NUMBER);
Table created.
SQL>
SQL> CREATE OR REPLACE TRIGGER before_statement_trigger
2 BEFORE INSERT ON to_table
3 BEGIN
4 DBMS_OUTPUT.PUT_LINE('Before Insert Statement Level');
5 END;
6 /
Trigger created.
SQL>
SQL> drop table to_table;
Table dropped.
SQL>
SQL>
SQL> CREATE TABLE to_table
2 (col1 NUMBER);
Table created.
SQL>
SQL> CREATE OR REPLACE TRIGGER before_row_trigger
2 BEFORE INSERT ON to_table
3 FOR EACH ROW
4 BEGIN
5 DBMS_OUTPUT.PUT_LINE('Before Insert Row Level');
6 END;
7 /
Trigger created.
SQL>
SQL> drop table to_table;
Table dropped.
SQL>