Merge pull request #217 from Trivadis/feature/issue-213-new-rule-for-loop

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;
+/
+```

docs/9-appendix/appendix.md

@@ -87,6 +87,7 @@ n/a | 4365 | Never use unconditional CONTINUE or EXIT in a loop. | Major |  |  |
 46 | 4375 | Always use EXIT WHEN instead of an IF statement to exit from a loop.  | Minor |  |  | ✘ |  |  |  |  | 
 47 | 4380 | Try to label your EXIT WHEN statements. | Minor |  |  | ✘ |  |  |  |  | 
 48 | 4385 | Never use a cursor for loop to check whether a cursor returns data. | Critical |  | ✘ |  |  |  |  |  | 
+n/a | 4387 | !!!CHARACTERISTIC ERROR!!! | Blocker |  | ✘ |  |  | ✘ |  |  | 
 49 | 4390 | Avoid use of unreferenced FOR loop indexes. | Major |  | ✘ |  |  |  |  |  | 
 50 | 4395 | Avoid hard-coded upper or lower bound values with FOR loops. | Minor | ✘ |  | ✘ |  |  |  |  | 
 n/a | 5010 | Try to use a error/logging framework for your application. | Critical |  |  |  |  | ✘ | ✘ |  | ✘