mysql-test/r/subselect.result:
  test suite of string & date types with group function in subselects
mysql-test/t/subselect.test:
  test suite of string & date types with group function in subselects
sql/item_subselect.cc:
  fixed bug in string type with group function
sql/item_subselect.h:
  fixed bug in string type with group function
sql/sql_class.cc:
  fixed bug in date type with group function