diff --git a/mysql-test/r/distinct.result b/mysql-test/r/distinct.result
index 00436019f85..a6ad95570f8 100644
--- a/mysql-test/r/distinct.result
+++ b/mysql-test/r/distinct.result
@@ -512,3 +512,24 @@ id	IFNULL(dsc, '-')
 2	line number two
 3	line number three
 drop table t1;
+CREATE TABLE t1 (
+ID int(11) NOT NULL auto_increment,
+x varchar(20) default NULL,
+y decimal(10,0) default NULL,
+PRIMARY KEY  (ID),
+KEY (y)
+) ENGINE=MyISAM DEFAULT CHARSET=latin1;
+INSERT INTO t1 VALUES
+(1,'ba','-1'),
+(2,'ba','1150'),
+(306,'ba','-1'),
+(307,'ba','1150'),
+(611,'ba','-1'),
+(612,'ba','1150');
+select count(distinct x,y) from t1;
+count(distinct x,y)
+2
+select count(distinct concat(x,y)) from t1;
+count(distinct concat(x,y))
+2
+drop table t1;
diff --git a/mysql-test/t/distinct.test b/mysql-test/t/distinct.test
index 6483284633f..45bd0c7a51c 100644
--- a/mysql-test/t/distinct.test
+++ b/mysql-test/t/distinct.test
@@ -358,3 +358,27 @@ select distinct id, IFNULL(dsc, '-') from t1;
 drop table t1;
 
 # End of 4.1 tests
+
+
+#
+# Bug #15745 ( COUNT(DISTINCT CONCAT(x,y)) returns wrong result)
+#
+CREATE TABLE t1 (
+  ID int(11) NOT NULL auto_increment,
+  x varchar(20) default NULL,
+  y decimal(10,0) default NULL,
+  PRIMARY KEY  (ID),
+  KEY (y)
+) ENGINE=MyISAM DEFAULT CHARSET=latin1;
+
+INSERT INTO t1 VALUES
+(1,'ba','-1'),
+(2,'ba','1150'),
+(306,'ba','-1'),
+(307,'ba','1150'),
+(611,'ba','-1'),
+(612,'ba','1150');
+
+select count(distinct x,y) from t1;
+select count(distinct concat(x,y)) from t1;
+drop table t1;
diff --git a/sql/item_sum.cc b/sql/item_sum.cc
index a3a25ec8d6f..804adcd022d 100644
--- a/sql/item_sum.cc
+++ b/sql/item_sum.cc
@@ -2541,9 +2541,9 @@ bool Item_sum_count_distinct::setup(THD *thd)
       Field *f= *field;
       enum enum_field_types type= f->type();
       tree_key_length+= f->pack_length();
-      if (!f->binary() && (type == MYSQL_TYPE_STRING ||
-                           type == MYSQL_TYPE_VAR_STRING ||
-                           type == MYSQL_TYPE_VARCHAR))
+      if ((type == MYSQL_TYPE_VARCHAR) ||
+          !f->binary() && (type == MYSQL_TYPE_STRING ||
+                           type == MYSQL_TYPE_VAR_STRING))
       {
         all_binary= FALSE;
         break;
diff --git a/sql/sql_lex.cc b/sql/sql_lex.cc
index 1ed8887a878..49b0c70ff03 100644
--- a/sql/sql_lex.cc
+++ b/sql/sql_lex.cc
@@ -223,7 +223,7 @@ static int find_keyword(LEX *lex, uint len, bool function)
 
   SYNOPSIS
     is_keyword()
-    name      checked name
+    name      checked name (must not be empty)
     len       length of checked name
 
   RETURN VALUES
@@ -233,6 +233,7 @@ static int find_keyword(LEX *lex, uint len, bool function)
 
 bool is_keyword(const char *name, uint len)
 {
+  DBUG_ASSERT(len != 0);
   return get_hash_symbol(name,len,0)!=0;
 }
 
diff --git a/sql/sql_show.cc b/sql/sql_show.cc
index 8920efa87ab..5c9d13cac3e 100644
--- a/sql/sql_show.cc
+++ b/sql/sql_show.cc
@@ -713,7 +713,8 @@ append_identifier(THD *thd, String *packet, const char *name, uint length)
 
 int get_quote_char_for_identifier(THD *thd, const char *name, uint length)
 {
-  if (!is_keyword(name,length) &&
+  if (!length ||
+      !is_keyword(name,length) &&
       !require_quotes(name, length) &&
       !(thd->options & OPTION_QUOTE_SHOW_CREATE))
     return EOF;