A fix and a test case for Bug#21002 "Derived table not selecting from a

"real" table fails in JOINs".

This is a regression caused by the fix for Bug 18444. 
This fix removed the assignment of empty_c_string to table->db performed 
in add_table_to_list, as neither me nor anyone else knew what it was 
there for. Now we know it and it's covered with tests: the only case 
when a table database name can be empty is when the table is a derived 
table. The fix puts the assignment back but makes it a bit more explicit.

Additionally, finally drop sp.result.orig which was checked in by mistake. 

mysql-test/r/derived.result

@@ -276,7 +276,7 @@ select * from t1;
 N	M
 3	0
 delete P1.*,p2.* from `t1` AS P1 INNER JOIN (SELECT N FROM `t1` GROUP BY N HAVING Count(M) > 1) AS p2 ON P1.N = p2.N;
-ERROR HY000: The target table p2 of the DELETE is not updatable
+ERROR 42S02: Unknown table 'p2' in MULTI DELETE
 delete P1.* from `t1` AS P1 INNER JOIN (SELECT aaa FROM `t1` GROUP BY N HAVING Count(M) > 1) AS P2 ON P1.N = P2.N;
 ERROR 42S22: Unknown column 'aaa' in 'field list'
 drop table t1;

mysql-test/r/sp.result

@@ -5057,4 +5057,19 @@ concat('data was: /', var1, '/')
 data was: /1/
 drop table t3|
 drop procedure bug15217|
+drop table if exists t3|
+drop database if exists mysqltest1|
+create table t3 (a int)|
+insert into t3 (a) values (1), (2)|
+create database mysqltest1|
+use mysqltest1|
+drop database mysqltest1|
+select database()|
+database()
+NULL
+select * from (select 1 as a) as t1 natural join (select * from test.t3) as t2|
+a
+1
+use test|
+drop table t3|
 drop table t1,t2;

mysql-test/t/derived.test

@@ -157,7 +157,7 @@ UPDATE `t1` AS P1 INNER JOIN (SELECT aaaa FROM `t1` GROUP BY N HAVING Count(M) >
 delete P1.* from `t1` AS P1 INNER JOIN (SELECT N FROM `t1` GROUP BY N HAVING Count(M) > 1) AS P2 ON P1.N = P2.N;
 select * from t1;
 --replace_result P2 p2
---error 1288
+--error ER_UNKNOWN_TABLE 
 delete P1.*,P2.* from `t1` AS P1 INNER JOIN (SELECT N FROM `t1` GROUP BY N HAVING Count(M) > 1) AS P2 ON P1.N = P2.N;
 -- error 1054
 delete P1.* from `t1` AS P1 INNER JOIN (SELECT aaa FROM `t1` GROUP BY N HAVING Count(M) > 1) AS P2 ON P1.N = P2.N;

mysql-test/t/sp.test

@@ -5962,6 +5962,31 @@ call bug15217()|
 drop table t3|
 drop procedure bug15217|
 
+#
+# Bug#21002 "Derived table not selecting from a "real" table fails in JOINs"
+#         
+# A regression caused by the fix for Bug#18444: for derived tables we should
+# set an empty string as the current database. They do not belong to any
+# database and must be usable even if there is no database
+# selected.
+--disable_warnings
+drop table if exists t3|
+drop database if exists mysqltest1|
+--enable_warnings
+create table t3 (a int)|
+insert into t3 (a) values (1), (2)|
+
+create database mysqltest1|
+use mysqltest1|
+drop database mysqltest1|
+
+# No current database
+select database()|
+
+select * from (select 1 as a) as t1 natural join (select * from test.t3) as t2|
+use test|
+drop table t3|
+
 #
 # BUG#NNNN: New bug synopsis
 #

sql/sp.cc

@@ -1846,7 +1846,6 @@ sp_use_new_db(THD *thd, LEX_STRING new_db, LEX_STRING *old_db,
 	      bool no_access_check, bool *dbchangedp)
 {
   int ret;
-  static char empty_c_string[1]= {0};          /* used for not defined db */
   DBUG_ENTER("sp_use_new_db");
   DBUG_PRINT("enter", ("newdb: %s", new_db.str));
 

sql/sql_class.cc

@@ -43,6 +43,7 @@
   table name
 */
 char internal_table_name[2]= "*";
+char empty_c_string[1]= {0};    /* used for not defined db */
 
 const char * const THD::DEFAULT_WHERE= "field list";
 

sql/sql_class.h

@@ -41,6 +41,7 @@ enum enum_check_fields { CHECK_FIELD_IGNORE, CHECK_FIELD_WARN,
 			 CHECK_FIELD_ERROR_FOR_NULL };
 
 extern char internal_table_name[2];
+extern char empty_c_string[1];
 extern const char **errmesg;
 
 #define TC_LOG_PAGE_SIZE   8192
@@ -1977,11 +1978,21 @@ public:
   {
     db.str=0;
   }
+  /*
+    This constructor is used only for the case when we create a derived
+    table. A derived table has no name and doesn't belong to any database.
+    Later, if there was an alias specified for the table, it will be set
+    by add_table_to_list.
+  */
   inline Table_ident(SELECT_LEX_UNIT *s) : sel(s)
   {
     /* We must have a table name here as this is used with add_table_to_list */
-    db.str=0; table.str= internal_table_name; table.length=1;
+    db.str= empty_c_string;                    /* a subject to casedn_str */
+    db.length= 0;
+    table.str= internal_table_name;
+    table.length=1;
   }
+  bool is_derived_table() const { return test(sel); }
   inline void change_db(char *db_name)
   {
     db.str= db_name; db.length= (uint) strlen(db_name);

sql/sql_parse.cc

@@ -6088,8 +6088,7 @@ TABLE_LIST *st_select_lex::add_table_to_list(THD *thd,
   if (!table)
     DBUG_RETURN(0);				// End of memory
   alias_str= alias ? alias->str : table->table.str;
-  if (check_table_name(table->table.str,table->table.length) ||
-      table->db.str && check_db_name(table->db.str))
+  if (check_table_name(table->table.str, table->table.length))
   {
     my_error(ER_WRONG_TABLE_NAME, MYF(0), table->table.str);
     DBUG_RETURN(0);
@@ -6110,6 +6109,11 @@ TABLE_LIST *st_select_lex::add_table_to_list(THD *thd,
     DBUG_RETURN(0);				/* purecov: inspected */
   if (table->db.str)
   {
+    if (table->is_derived_table() == FALSE && check_db_name(table->db.str))
+    {
+      my_error(ER_WRONG_DB_NAME, MYF(0), table->db.str);
+      DBUG_RETURN(0);
+    }
     ptr->db= table->db.str;
     ptr->db_length= table->db.length;
   }