add G-4387: Never use a FOR LOOP for a query that should return not more than one row.

Author: PhilippSalvisberg

docs/4-language-usage/4-control-structures/3-flow-control/g-4387.md

@@ -0,0 +1,58 @@
+# G-4387: Never use a FOR LOOP for a query that should return not more than one row.
+
+!!! bug "Blocker"
+    Reliability, Efficiency, Readability
+
+!!! missing "Unsupported in db\* CODECOP Validators"
+    Without access to the Oracle Data Dictionary, we cannot determine the number of rows to be processed.
+    
+## Reason
+
+A `for loop` can hide a `too_many_rows` exception. The more complex a query is, the higher is the risk that more than one row will be processed.
+This affects performance and can lead to a wrong result. 
+
+A `for loop` can also hide a `no_data_found` exception and the reader cannot determine whether this is intentional or not.
+
+## Example (bad)
+
+``` sql
+create or replace package body employee_api is
+   function emp_name(in_empno in integer) return varchar2 is -- NOSONAR: non-deterministic
+      l_ename emp.ename%type;
+   begin
+      <<fetch_name>>
+      for r in (
+         select ename
+           from emp
+          where empno = in_empno
+      )
+      loop
+         l_ename := r.ename;
+      end loop fetch_name;
+      return l_ename;
+   end emp_name;
+end employee_api;
+/
+```
+
+## Example (good)
+
+``` sql
+create or replace package body employee_api is
+   function emp_name(in_empno in integer) return varchar2 is -- NOSONAR: non-deterministic
+      l_ename emp.ename%type;
+   begin
+      select ename
+        into l_ename
+        from emp
+       where empno >= in_empno;
+      return l_ename;
+   exception
+      when no_data_found then
+         return null;
+      when too_many_rows then
+         raise;
+   end emp_name;
+end employee_api;
+/
+```