COALESCE now aggregates its argument types in this way:

if some of the arguments is STRING_RESULT the STRING_RESULT
else if some of the arguments is REAL_RESULT then REAL_RESULT
else INT_RESULT 

mysql-test/r/case.result

@@ -71,3 +71,24 @@ orange
 yellow
 green
 drop table t1;
+SET NAMES latin1;
+CREATE TABLE t1 SELECT COALESCE(_latin1'a',_latin2'a');
+ERROR HY000: Illegal mix of collations (latin1_swedish_ci,COERCIBLE) and (latin2_general_ci,COERCIBLE) for operation 'coalesce'
+CREATE TABLE t1 SELECT COALESCE('a' COLLATE latin1_swedish_ci,'b' COLLATE latin1_bin);
+ERROR HY000: Illegal mix of collations (latin1_swedish_ci,EXPLICIT) and (latin1_bin,EXPLICIT) for operation 'coalesce'
+CREATE TABLE t1 SELECT 
+COALESCE(1), COALESCE(1.0),COALESCE('a'),
+COALESCE(1,1.0), COALESCE(1,'1'),COALESCE(1.1,'1'),
+COALESCE('a' COLLATE latin1_bin,'b');
+SHOW CREATE TABLE t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `COALESCE(1)` int(1) NOT NULL default '0',
+  `COALESCE(1.0)` double(3,1) NOT NULL default '0.0',
+  `COALESCE('a')` char(1) NOT NULL default '',
+  `COALESCE(1,1.0)` double(3,1) NOT NULL default '0.0',
+  `COALESCE(1,'1')` char(1) NOT NULL default '',
+  `COALESCE(1.1,'1')` char(3) NOT NULL default '',
+  `COALESCE('a' COLLATE latin1_bin,'b')` char(1) character set latin1 collate latin1_bin NOT NULL default ''
+) TYPE=MyISAM CHARSET=latin1
+DROP TABLE t1;

mysql-test/t/case.test

@@ -41,3 +41,25 @@ create table t1 (row int not null, col int not null, val varchar(255) not null);
 insert into t1 values (1,1,'orange'),(1,2,'large'),(2,1,'yellow'),(2,2,'medium'),(3,1,'green'),(3,2,'small');
 select max(case col when 1 then val else null end) as color from t1 group by row;
 drop table t1;
+
+#
+# COALESCE is a CASE abbrevation:
+#
+# COALESCE(v1,v2) == CASE WHEN v1 IS NOT NULL THEN v1 ELSE v2 END
+#
+# COALESCE(V1, V2, . . . ,Vn ) =  
+#     CASE WHEN V1 IS NOT NULL THEN V1 ELSE COALESCE (V2, . . . ,Vn) END
+#
+# Check COALESCE argument types aggregation
+
+SET NAMES latin1;
+--error 1265
+CREATE TABLE t1 SELECT COALESCE(_latin1'a',_latin2'a');
+--error 1265
+CREATE TABLE t1 SELECT COALESCE('a' COLLATE latin1_swedish_ci,'b' COLLATE latin1_bin);
+CREATE TABLE t1 SELECT 
+ COALESCE(1), COALESCE(1.0),COALESCE('a'),
+ COALESCE(1,1.0), COALESCE(1,'1'),COALESCE(1.1,'1'),
+ COALESCE('a' COLLATE latin1_bin,'b');
+SHOW CREATE TABLE t1;
+DROP TABLE t1;

sql/item_cmpfunc.cc

@@ -1149,7 +1149,13 @@ void Item_func_coalesce::fix_length_and_dec()
   {
     set_if_bigger(max_length,args[i]->max_length);
     set_if_bigger(decimals,args[i]->decimals);
+    cached_result_type=item_store_type(cached_result_type,
+				       args[i]->result_type());
   }
+  if (cached_result_type == STRING_RESULT)
+    agg_arg_collations(collation, args, arg_count);
+  else if (cached_result_type != REAL_RESULT)
+    decimals= 0;
 }
 
 /****************************************************************************