Fixed bug #29360.

The special `zero' enum value was coerced to the normal
empty string enum value during a field-to-field copy.
This bug affected CREATE ... SELECT statements and
SELECT aggregate GROUP BY enum field statements.

Also this bug made unnecessary warnings during
the execution of CREATE ... SELECT statements:
Warning       1265    Data truncated for column...

1 files changed, 11 insertions, 5 deletions

diff --git a/sql/field_conv.cc b/sql/field_conv.cc
index a286255ec23..2df65b62de6 100644
--- a/sql/field_conv.cc
+++ b/sql/field_conv.cc
@@ -790,11 +790,17 @@ int field_conv(Field *to,Field *from)
       blob->value.copy();
     return blob->store(blob->value.ptr(),blob->value.length(),from->charset());
   }
-  if ((from->result_type() == STRING_RESULT &&
-       (to->result_type() == STRING_RESULT ||
-	(from->real_type() != FIELD_TYPE_ENUM &&
-	 from->real_type() != FIELD_TYPE_SET))) ||
-      to->type() == FIELD_TYPE_DECIMAL)
+  if (from->real_type() == FIELD_TYPE_ENUM &&
+      to->real_type() == FIELD_TYPE_ENUM &&
+      from->val_int() == 0)
+  {
+    ((Field_enum *)(to))->store_type(0);
+  }
+  else if ((from->result_type() == STRING_RESULT &&
+            (to->result_type() == STRING_RESULT ||
+             (from->real_type() != FIELD_TYPE_ENUM &&
+              from->real_type() != FIELD_TYPE_SET))) ||
+           to->type() == FIELD_TYPE_DECIMAL)
   {
     char buff[MAX_FIELD_WIDTH];
     String result(buff,sizeof(buff),from->charset());