Bug#32848: Data type conversion bug in union subselects in MySQL 5.0.38

There were two problems when inferring the correct field types resulting from
UNION queries.
- If the type is NULL for all corresponding fields in the UNION, the resulting 
  type would be NULL, while the type is BINARY(0) if there is just a single 
  SELECT NULL.
- If one SELECT in the UNION uses a subselect, a temporary table is created
  to represent the subselect, and the result type defaults to a STRING type,
  hiding the fact that the type was unknown(just a NULL value).
Fixed by remembering whenever a field was created from a NULL value and pass
type NULL to the type coercion if that is the case, and creating a string field
as result of UNION only if the type would otherwise be NULL.


mysql-test/r/union.result:
  Bug#32848: Test result
mysql-test/t/union.test:
  Bug#32848: Test case
sql/field.cc:
  Bug#32848: Initialization of new field
sql/field.h:
  Bug#32848: New member to record when a field was created from a NULL value.
sql/item.cc:
  Bug#32848: 
  A field created from a NULL value will submit NULL as type to the 
  type coercion procedure.
  If Item_type_holder has not inferred the correct type after processing all
  SELECTs in a UNION, a string field is created.
sql/sql_select.cc:
  Bug#32848: Recording when a field is created from a NULL value.

1 files changed, 24 insertions, 0 deletions

diff --git a/mysql-test/t/union.test b/mysql-test/t/union.test
index 22f09466b1c..84616aa281d 100644
--- a/mysql-test/t/union.test
+++ b/mysql-test/t/union.test
@@ -877,4 +877,28 @@ DROP TABLE t1;
 select @var;
 --error 1172
 (select 2) union (select 1 into @var);
+
+#
+# Bug#32848: Data type conversion bug in union subselects in MySQL 5.0.38
+#
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (1), (2), (3);
+
+CREATE TABLE t2 SELECT * FROM (SELECT NULL) a UNION SELECT a FROM t1;
+DESC t2;
+
+CREATE TABLE t3 SELECT a FROM t1 UNION SELECT * FROM (SELECT NULL) a;
+DESC t3;
+
+CREATE TABLE t4 SELECT NULL;
+DESC t4;
+
+CREATE TABLE t5 SELECT NULL UNION SELECT NULL;
+DESC t5;
+
+CREATE TABLE t6 
+SELECT * FROM (SELECT * FROM (SELECT NULL)a) b UNION SELECT a FROM t1;
+DESC t6;
+
+DROP TABLE t1, t2, t3, t4, t5, t6;
 --echo End of 5.0 tests