select.result, select.test:

  Added a test case for bug #7098.
sql_select.cc:
  Fixed bug #7098.
  When a string field was substituted for an equal constant
  the collation of the constant was changed by mistake for 
  the binary collation.

mysql-test/r/select.result

@@ -2400,3 +2400,25 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	5	
 1	SIMPLE	t2	ref	a	a	23	test.t1.a	2	
 DROP TABLE t1, t2;
+CREATE TABLE t1 ( city char(30) );
+INSERT INTO t1 VALUES ('London');
+INSERT INTO t1 VALUES ('Paris');
+SELECT * FROM t1 WHERE city='London';
+city
+London
+SELECT * FROM t1 WHERE city='london';
+city
+London
+EXPLAIN SELECT * FROM t1 WHERE city='London' AND city='london';
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	2	Using where
+SELECT * FROM t1 WHERE city='London' AND city='london';
+city
+London
+EXPLAIN SELECT * FROM t1 WHERE city LIKE '%london%' AND city='London';
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	2	Using where
+SELECT * FROM t1 WHERE city LIKE '%london%' AND city='London';
+city
+London
+DROP TABLE t1;

mysql-test/t/select.test

@@ -1945,3 +1945,22 @@ EXPLAIN SELECT * FROM t1 LEFT JOIN t2 USE INDEX (a) ON t1.a=t2.a;
 EXPLAIN SELECT * FROM t1 LEFT JOIN t2 FORCE INDEX (a) ON t1.a=t2.a;
 
 DROP TABLE t1, t2;
+
+
+#
+# Test case for bug 7098: substitution of a constant for a string field 
+#
+
+CREATE TABLE t1 ( city char(30) );
+INSERT INTO t1 VALUES ('London');
+INSERT INTO t1 VALUES ('Paris');
+
+SELECT * FROM t1 WHERE city='London';
+SELECT * FROM t1 WHERE city='london';
+EXPLAIN SELECT * FROM t1 WHERE city='London' AND city='london';
+SELECT * FROM t1 WHERE city='London' AND city='london';
+EXPLAIN SELECT * FROM t1 WHERE city LIKE '%london%' AND city='London';
+SELECT * FROM t1 WHERE city LIKE '%london%' AND city='London';
+
+DROP TABLE t1;
+

sql/sql_select.cc

@@ -4230,6 +4230,8 @@ change_cond_ref_to_const(THD *thd, I_List<COND_CMP> *save_list,
     Item *tmp=value->new_item();
     if (tmp)
     {
+      tmp->collation.set(value->collation.collation,
+                         value->collation.derivation);
       thd->change_item_tree(args + 1, tmp);
       func->update_used_tables();
       if ((functype == Item_func::EQ_FUNC || functype == Item_func::EQUAL_FUNC)
@@ -4251,6 +4253,8 @@ change_cond_ref_to_const(THD *thd, I_List<COND_CMP> *save_list,
     Item *tmp=value->new_item();
     if (tmp)
     {
+      tmp->collation.set(value->collation.collation,
+                         value->collation.derivation);
       thd->change_item_tree(args, tmp);
       value= tmp;
       func->update_used_tables();