func_gconcat.test, func_gconcat.result:

  Test case for bug #12859 group_concat in subquery cause incorrect not null.

mysql-test/r/func_gconcat.result

@@ -551,3 +551,12 @@ DROP TABLE t1,t2;
 select * from (select group_concat('c') from DUAL) t;
 group_concat('c')
 NULL
+create table t1 ( a int not null default 0);
+select * from (select group_concat(a) from t1) t2;
+group_concat(a)
+NULL
+select group_concat('x') UNION ALL select 1;
+group_concat('x')
+NULL
+1
+drop table t1;

mysql-test/t/func_gconcat.test

@@ -348,4 +348,12 @@ DROP TABLE t1,t2;
 #
 select * from (select group_concat('c') from DUAL) t;
 
+#
+# Bug #12859 group_concat in subquery cause incorrect not null
+#
+create table t1 ( a int not null default 0);
+select * from (select group_concat(a) from t1) t2;
+select group_concat('x') UNION ALL select 1;
+drop table t1;
+
 # End of 4.1 tests