Index: java/engine/org/apache/derby/impl/sql/compile/TernaryOperatorNode.java
===================================================================
--- java/engine/org/apache/derby/impl/sql/compile/TernaryOperatorNode.java	(revision 468290)
+++ java/engine/org/apache/derby/impl/sql/compile/TernaryOperatorNode.java	(working copy)
@@ -921,10 +921,18 @@
     	if (isSameNodeType(o)) 
 	{
 		TernaryOperatorNode other = (TernaryOperatorNode)o;
+		
+			/*
+			 * SUBSTR function can either have 2 or 3 arguments.  In the 
+			 * 2-args case, rightOperand will be null and thus needs 
+			 * additional handling in the equivalence check.
+			 */
     		return (other.methodName.equals(methodName)
 				&& other.receiver.isEquivalent(receiver)
     				&& other.leftOperand.isEquivalent(leftOperand)
-    				&& other.rightOperand.isEquivalent(rightOperand));
+    				&& ( (rightOperand == null && other.rightOperand == null) || 
+    				     (other.rightOperand != null && 
+    				    	other.rightOperand.isEquivalent(rightOperand)) ) );
         }
     	return false;
     }
Index: java/testing/org/apache/derbyTesting/functionTests/tests/lang/groupBy.sql
===================================================================
--- java/testing/org/apache/derbyTesting/functionTests/tests/lang/groupBy.sql	(revision 468290)
+++ java/testing/org/apache/derbyTesting/functionTests/tests/lang/groupBy.sql	(working copy)
@@ -360,3 +360,15 @@
 -- ok, gives one row
 select 10,avg(c) from t having 1 < 2;
 drop table t;
+
+-- DERBY-2008
+-- test SUBSTR with 2 args with GROUP BY expression
+create table dt (vc varchar(30));
+insert into dt values ('1928-09-21'), ('1903-12-08');
+-- ok
+select substr(vc, 3) from dt group by substr(vc, 3); 
+select substr(vc, 3, 4) from dt group by substr(vc, 3, 4); 
+-- expect errors
+select substr(vc, 3, 4) from dt group by substr(vc, 3); 
+select substr(vc, 3) from dt group by substr(vc, 3, 4);
+drop table dt;
Index: java/testing/org/apache/derbyTesting/functionTests/master/groupBy.out
===================================================================
--- java/testing/org/apache/derbyTesting/functionTests/master/groupBy.out	(revision 468290)
+++ java/testing/org/apache/derbyTesting/functionTests/master/groupBy.out	(working copy)
@@ -636,4 +636,28 @@
 10         |2          
 ij> drop table t;
 0 rows inserted/updated/deleted
+ij> -- DERBY-2008
+-- test SUBSTR with 2 args with GROUP BY expression
+create table dt (vc varchar(30));
+0 rows inserted/updated/deleted
+ij> insert into dt values ('1928-09-21'), ('1903-12-08');
+2 rows inserted/updated/deleted
+ij> -- ok
+select substr(vc, 3) from dt group by substr(vc, 3);
+1                             
+------------------------------
+03-12-08                      
+28-09-21                      
+ij> select substr(vc, 3, 4) from dt group by substr(vc, 3, 4);
+1   
+----
+03-1
+28-0
+ij> -- expect errors
+select substr(vc, 3, 4) from dt group by substr(vc, 3);
+ERROR 42Y30: The SELECT list of a grouped query contains at least one invalid expression. If a SELECT list has a GROUP BY, the list may only contain valid grouping expressions and valid aggregate expressions.  
+ij> select substr(vc, 3) from dt group by substr(vc, 3, 4);
+ERROR 42Y30: The SELECT list of a grouped query contains at least one invalid expression. If a SELECT list has a GROUP BY, the list may only contain valid grouping expressions and valid aggregate expressions.  
+ij> drop table dt;
+0 rows inserted/updated/deleted
 ij>