Move all sql_type() to the same file

sql/field.cc

@@ -317,6 +317,16 @@ bool Field::optimize_range(uint idx)
   return test(table->file->index_flags(idx) & HA_READ_NEXT);
 }
 
+/****************************************************************************
+  Functions for the Field_null
+****************************************************************************/
+
+void Field_null::sql_type(String &res) const
+{
+  res.set("null",4,system_charset_info);
+}
+
+
 /****************************************************************************
   Functions for the Field_decimal class
   This is an number stored as a pre-space (or pre-zero) string

sql/field.h

@@ -556,7 +556,7 @@ class Field_null :public Field_str {
   int cmp(const char *a, const char *b) { return 0;}
   void sort_string(char *buff, uint length)  {}
   uint32 pack_length() const { return 0; }
-  void sql_type(String &str) const { str.set("null",4,my_thd_charset); }
+  void sql_type(String &str) const;
   uint size_of() const { return sizeof(*this); }
 };