Check out example codes for "creation d'un trigger sql". It will help you in understanding the concepts better.

Code Example 1

CREATE OR REPLACE TRIGGER trigg_example
 BEFORE INSERT OR UPDATE ON table_example
 FOR EACH ROW
 WHEN (new.no_line > 0)
 DECLARE
     evol_exemple number;
 BEGIN
     evol_exemple := :new.exemple  - :old.exemple;
     DBMS_OUTPUT.PUT_LINE('  evolution : ' || evol_exemple);
 END;

Learn ReactJs, React Native from akashmittal.com