func_if.result, func_if.test:

  Added a test case for bug #11142.
item_cmpfunc.cc:
  Fixed bug #11142.
  Implementation of Item_func_nullif::is_null was corrected.

mysql-test/r/func_if.result

@@ -86,3 +86,30 @@ drop table t1;
 SELECT NULLIF(5,5) IS NULL, NULLIF(5,5) IS NOT NULL;
 NULLIF(5,5) IS NULL	NULLIF(5,5) IS NOT NULL
 1	0
+DROP TABLE IF EXISTS t1;
+CREATE TABLE t1 (a CHAR(10));
+INSERT INTO t1 VALUES ('aaa'), (NULL), (''), ('bbb');
+SELECT a, NULLIF(a,'') FROM t1;
+a	NULLIF(a,'')
+aaa	aaa
+NULL	NULL
+	NULL
+bbb	bbb
+SELECT a, NULLIF(a,'') FROM t1 WHERE NULLIF(a,'') IS NULL;
+a	NULLIF(a,'')
+NULL	NULL
+	NULL
+DROP TABLE t1;
+CREATE TABLE t1 (a CHAR(10));
+INSERT INTO t1 VALUES ('aaa'), (NULL), (''), ('bbb');
+SELECT a, NULLIF(a,'') FROM t1;
+a	NULLIF(a,'')
+aaa	aaa
+NULL	NULL
+	NULL
+bbb	bbb
+SELECT a, NULLIF(a,'') FROM t1 WHERE NULLIF(a,'') IS NULL;
+a	NULLIF(a,'')
+NULL	NULL
+	NULL
+DROP TABLE t1;

mysql-test/t/func_if.test

@@ -60,3 +60,30 @@ drop table t1;
 # Bug #5595  NULLIF() IS NULL returns false if NULLIF() returns NULL
 #
 SELECT NULLIF(5,5) IS NULL, NULLIF(5,5) IS NOT NULL;
+#
+# Test to check evaluation of MULLIF when the first argument is NULL
+# (motivated by the Bug 11142)
+#
+
+--disable_warnings
+DROP TABLE IF EXISTS t1;
+--enable_warnings
+
+CREATE TABLE t1 (a CHAR(10));
+INSERT INTO t1 VALUES ('aaa'), (NULL), (''), ('bbb');
+SELECT a, NULLIF(a,'') FROM t1;
+SELECT a, NULLIF(a,'') FROM t1 WHERE NULLIF(a,'') IS NULL;
+
+DROP TABLE t1;
+
+#
+# Test for bug #11142: evaluation of NULLIF when the first argument is NULL
+#
+
+CREATE TABLE t1 (a CHAR(10));
+INSERT INTO t1 VALUES ('aaa'), (NULL), (''), ('bbb');
+
+SELECT a, NULLIF(a,'') FROM t1;
+SELECT a, NULLIF(a,'') FROM t1 WHERE NULLIF(a,'') IS NULL;
+
+DROP TABLE t1;

sql/item_cmpfunc.cc

@@ -1161,9 +1161,7 @@ Item_func_nullif::val_str(String *str)
 bool
 Item_func_nullif::is_null()
 {
-  if (!cmp.compare())
-    return (null_value=1);
-  return 0;
+  return (null_value= (!cmp.compare() ? 1 : args[0]->null_value)); 
 }
 
 /*