After missing to add a foreign key to a table, I ended up having to correct my mistake in many places in my PL SQL code.
Afterwards I thought: "Who created this table without this important constraint?! ... :0( Uups! It was me."
If something similar happens to you, below you'll find an SQL to add a foreign key to a table.
'References parent_table_name(parent_field)' gives the parent table, while 'Foreign Key(field_name)' is the field from the child table, which will be referenced.
If you receive an error message saying that the alter table constraint could not be validated because the table has child records, one quick fix is to delete all records in child and parent table and then executing the alter table statement again.
/* Drop a table constraint */
ALTER TABLE sod_cat_app_cat
fk_sod_category_app; /* Name of a constraint */
/*Add a Table Constraints for foreign key*/
Alter Table sod_cat_app_cat Modify (