fix for a bug with UNION in sub-select's and start working on

UNION's in derived tables

mysql-test/r/union.result

@@ -201,3 +201,6 @@ set SQL_SELECT_LIMIT=DEFAULT;
 drop table t1,t2;
 select * union select 1;
 No tables used
+select 1 as a,(select a union select a);
+a	(select a union select a)
+1	1

mysql-test/t/union.test

@@ -103,3 +103,4 @@ drop table t1,t2;
 
 --error 1096
 select * union select 1;
+select 1 as a,(select a union select a);

sql/sql_union.cc

@@ -125,6 +125,8 @@ int st_select_lex_unit::prepare(THD *thd, select_result *result)
   SELECT_LEX_NODE *lex_select_save= thd->lex.current_select;
   SELECT_LEX *sl;
 
+  if (lex_select_save->linkage != DERIVED_TABLE_TYPE)
+    thd->lex.current_select=first_select();
   /* Global option */
   if (((void*)(global_parameters)) == ((void*)this))
   {

sql/sql_yacc.yy

@@ -2504,7 +2504,7 @@ select_derived:
 	  mysql_init_select(lex);
 	  lex->current_select->linkage= DERIVED_TABLE_TYPE;
 	}
-        select_options select_item_list opt_select_from
+        select_options select_item_list opt_select_from union_opt
         ;
 
 opt_outer: