fixes #61 - explain use of ALWAYS in good example

Author: PhilippSalvisberg

docs/4-language-usage/3-dml-and-sql/1-general/g-3150.md

@@ -43,4 +43,8 @@ CREATE TABLE locations (
   ,city               VARCHAR2(30 CHAR) NOT NULL
   ,CONSTRAINT locations_pk PRIMARY KEY (location_id))
 /
-```
+```
+
+`GENERATED ALWAYS AS IDENTITY` ensures that the `location_id` is populated by a sequence. It is not possible to override the behavior in the application. 
+
+However, if you use a framework that produces an `INSERT` statement including the surrogate key column, and you cannot change this behavior, then you have to use the `GENERATED BY DEFAULT ON NULL AS IDENTITY` option. This has the downside that the application may pass a value, which might lead to an immediate or delayed `ORA-00001: unique constraint violated` error.