SELECT Statements Within UDRs in the Select List

A Select trigger is activated by a UDR if the UDR contains a SELECT statement within its statement block, and the UDR also appears in the select list of the projection clause of a SELECT statement. For example, assume that a UDR named my_rtn contains this SELECT statement in its statement block:

SELECT col1 FROM tab1

Now suppose that the following SELECT statement invokes the my_rtn UDR in its select list:

SELECT my_rtn() FROM tab2

This SELECT statement activates the Select trigger defined on column col1 of table tab1 when the my_rtn UDR is executed.