miguel@txg:~$ dbs/5.0/bin/mysql -uroot
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 2                            
Server version: 5.0.82-debug Source distribution         

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> -- original test case with primary key
mysql> drop database if exists db1;          
Query OK, 4 rows affected (0.01 sec)         

mysql> create database db1;
Query OK, 1 row affected (0.00 sec)

mysql> use db1
Database changed
mysql> create table foo (id int primary key, mystring char(25)) ENGINE=MyISAM;
Query OK, 0 rows affected (0.02 sec)                                          

mysql> create table bar (id int primary key, mystring char(25)) ENGINE=MyISAM;
Query OK, 0 rows affected (0.02 sec)                                          

mysql> create table baz (id int primary key, mystring char(25)) ENGINE=MyISAM;
Query OK, 0 rows affected (0.01 sec)                                          

mysql> insert into foo values (1, 'foo');
Query OK, 1 row affected (0.00 sec)      

mysql> insert into bar values (1, 'bar');
Query OK, 1 row affected (0.01 sec)      

mysql> insert into baz values (1, 'baz');
Query OK, 1 row affected (0.00 sec)      

mysql> create table qux (id int primary key, mystring char(25)) ENGINE=MRG_MyISAM
    -> UNION=(foo,bar,baz);                                                      
Query OK, 0 rows affected (0.01 sec)                                             

mysql> select distinct id from qux;
+----+                             
| id |                             
+----+                             
|  1 |                             
|  1 |                             
|  1 |                             
+----+                             
3 rows in set (0.00 sec)           

mysql> 
mysql> -- test case without primary key
mysql> drop database if exists db1;    
Query OK, 4 rows affected (0.01 sec)   

mysql> create database db1;
Query OK, 1 row affected (0.00 sec)

mysql> use db1
Database changed
mysql> create table foo (id int, mystring char(25)) ENGINE=MyISAM;
Query OK, 0 rows affected (0.14 sec)                              

mysql> create table bar (id int, mystring char(25)) ENGINE=MyISAM;
Query OK, 0 rows affected (0.06 sec)                              

mysql> create table baz (id int, mystring char(25)) ENGINE=MyISAM;
Query OK, 0 rows affected (0.04 sec)

mysql> insert into foo values (1, 'foo');
Query OK, 1 row affected (0.00 sec)

mysql> insert into bar values (1, 'bar');
Query OK, 1 row affected (0.00 sec)

mysql> insert into baz values (1, 'baz');
Query OK, 1 row affected (0.01 sec)

mysql> create table qux (id int, mystring char(25)) ENGINE=MRG_MyISAM
    -> UNION=(foo,bar,baz);
Query OK, 0 rows affected (0.03 sec)

mysql> select distinct id from qux;
+------+
| id   |
+------+
|    1 |
+------+
1 row in set (0.00 sec)

Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 1                            
Server version: 5.1.35-debug Source distribution         

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> -- original test case with primary key
mysql> drop database if exists db1;          
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> create database db1;
Query OK, 1 row affected (0.00 sec)

mysql> use db1
Database changed
mysql> create table foo (id int primary key, mystring char(25)) ENGINE=MyISAM;
Query OK, 0 rows affected (0.03 sec)                                          

mysql> create table bar (id int primary key, mystring char(25)) ENGINE=MyISAM;
Query OK, 0 rows affected (0.02 sec)                                          

mysql> create table baz (id int primary key, mystring char(25)) ENGINE=MyISAM;
Query OK, 0 rows affected (0.01 sec)                                          

mysql> insert into foo values (1, 'foo');
Query OK, 1 row affected (0.00 sec)      

mysql> insert into bar values (1, 'bar');
Query OK, 1 row affected (0.00 sec)      

mysql> insert into baz values (1, 'baz');
Query OK, 1 row affected (0.01 sec)      

mysql> create table qux (id int primary key, mystring char(25)) ENGINE=MRG_MyISAM
    -> UNION=(foo,bar,baz);                                                      
Query OK, 0 rows affected (0.03 sec)                                             

mysql> select distinct id from qux;
+----+                             
| id |                             
+----+                             
|  1 |                             
|  1 |                             
|  1 |                             
+----+                             
3 rows in set (0.00 sec)           

mysql> 
mysql> -- test case without primary key
mysql> drop database if exists db1;    
Query OK, 4 rows affected (0.04 sec)   

mysql> create database db1;
Query OK, 1 row affected (0.00 sec)

mysql> use db1
Database changed
mysql> create table foo (id int, mystring char(25)) ENGINE=MyISAM;
Query OK, 0 rows affected (0.02 sec)                              

mysql> create table bar (id int, mystring char(25)) ENGINE=MyISAM;
Query OK, 0 rows affected (0.02 sec)                              

mysql> create table baz (id int, mystring char(25)) ENGINE=MyISAM;
Query OK, 0 rows affected (0.02 sec)

mysql> insert into foo values (1, 'foo');
Query OK, 1 row affected (0.00 sec)

mysql> insert into bar values (1, 'bar');
Query OK, 1 row affected (0.00 sec)

mysql> insert into baz values (1, 'baz');
Query OK, 1 row affected (0.01 sec)

mysql> create table qux (id int, mystring char(25)) ENGINE=MRG_MyISAM
    -> UNION=(foo,bar,baz);
Query OK, 0 rows affected (0.03 sec)

mysql> select distinct id from qux;
+------+
| id   |
+------+
|    1 |
+------+
1 row in set (0.01 sec)

Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 1                            
Server version: 6.0.11-alpha-debug Source distribution   

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> -- original test case with primary key
mysql> drop database if exists db1;          
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> create database db1;
Query OK, 1 row affected (0.00 sec)

mysql> use db1
Database changed
mysql> create table foo (id int primary key, mystring char(25)) ENGINE=MyISAM;
Query OK, 0 rows affected (0.14 sec)                                          

mysql> create table bar (id int primary key, mystring char(25)) ENGINE=MyISAM;
Query OK, 0 rows affected (0.09 sec)                                          

mysql> create table baz (id int primary key, mystring char(25)) ENGINE=MyISAM;
Query OK, 0 rows affected (0.02 sec)                                          

mysql> insert into foo values (1, 'foo');
Query OK, 1 row affected (0.01 sec)      

mysql> insert into bar values (1, 'bar');
Query OK, 1 row affected (0.00 sec)      

mysql> insert into baz values (1, 'baz');
Query OK, 1 row affected (0.01 sec)      

mysql> create table qux (id int primary key, mystring char(25)) ENGINE=MRG_MyISAM
    -> UNION=(foo,bar,baz);                                                      
Query OK, 0 rows affected (0.08 sec)                                             

mysql> select distinct id from qux;
+----+                             
| id |                             
+----+                             
|  1 |                             
|  1 |                             
|  1 |                             
+----+                             
3 rows in set (0.01 sec)           

mysql> 
mysql> -- test case without primary key
mysql> drop database if exists db1;    
Query OK, 4 rows affected (0.08 sec)   

mysql> create database db1;
Query OK, 1 row affected (0.00 sec)

mysql> use db1
Database changed
mysql> create table foo (id int, mystring char(25)) ENGINE=MyISAM;
Query OK, 0 rows affected (0.04 sec)                              

mysql> create table bar (id int, mystring char(25)) ENGINE=MyISAM;
Query OK, 0 rows affected (0.05 sec)                              

mysql> create table baz (id int, mystring char(25)) ENGINE=MyISAM;
Query OK, 0 rows affected (0.01 sec)

mysql> insert into foo values (1, 'foo');
Query OK, 1 row affected (0.00 sec)

mysql> insert into bar values (1, 'bar');
Query OK, 1 row affected (0.01 sec)

mysql> insert into baz values (1, 'baz');
Query OK, 1 row affected (0.01 sec)

mysql> create table qux (id int, mystring char(25)) ENGINE=MRG_MyISAM
    -> UNION=(foo,bar,baz);
Query OK, 0 rows affected (0.03 sec)

mysql> select distinct id from qux;
+------+
| id   |
+------+
|    1 |
+------+
1 row in set (0.00 sec)

D:\mysql-4.1.22-win32>bin\mysql -uroot
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 1 to server version: 4.1.22-community

Type 'help;' or '\h' for help. Type '\c' to clear the buffer.

mysql> -- original test case with primary key
mysql> drop database if exists db1;
Query OK, 0 rows affected, 1 warning (0.04 sec)

mysql> create database db1;
Query OK, 1 row affected (0.05 sec)

mysql> use db1
Database changed
mysql> create table foo (id int primary key, mystring char(25)) ENGINE=MyISAM;
Query OK, 0 rows affected (0.09 sec)

mysql> create table bar (id int primary key, mystring char(25)) ENGINE=MyISAM;
Query OK, 0 rows affected (0.07 sec)

mysql> create table baz (id int primary key, mystring char(25)) ENGINE=MyISAM;
Query OK, 0 rows affected (0.08 sec)

mysql> insert into foo values (1, 'foo');
Query OK, 1 row affected (0.02 sec)

mysql> insert into bar values (1, 'bar');
Query OK, 1 row affected (0.00 sec)

mysql> insert into baz values (1, 'baz');
Query OK, 1 row affected (0.00 sec)

mysql> create table qux (id int primary key, mystring char(25)) ENGINE=MRG_MyISAM
    -> UNION=(foo,bar,baz);
Query OK, 0 rows affected (0.06 sec)

mysql> select distinct id from qux;
+----+
| id |
+----+
|  1 |
|  1 |
|  1 |
+----+
3 rows in set (0.01 sec)

mysql>
mysql> -- test case without primary key
mysql> drop database if exists db1;
Query OK, 4 rows affected (0.15 sec)

mysql> create database db1;
Query OK, 1 row affected (0.01 sec)

mysql> use db1
Database changed
mysql> create table foo (id int, mystring char(25)) ENGINE=MyISAM;
Query OK, 0 rows affected (0.13 sec)

mysql> create table bar (id int, mystring char(25)) ENGINE=MyISAM;
Query OK, 0 rows affected (0.05 sec)

mysql> create table baz (id int, mystring char(25)) ENGINE=MyISAM;
Query OK, 0 rows affected (0.09 sec)

mysql> insert into foo values (1, 'foo');
Query OK, 1 row affected (0.00 sec)

mysql> insert into bar values (1, 'bar');
Query OK, 1 row affected (0.00 sec)

mysql> insert into baz values (1, 'baz');
Query OK, 1 row affected (0.00 sec)

mysql> create table qux (id int, mystring char(25)) ENGINE=MRG_MyISAM
    -> UNION=(foo,bar,baz);
Query OK, 0 rows affected (0.07 sec)

mysql> select distinct id from qux;
+------+
| id   |
+------+
|    1 |
+------+
1 row in set (0.02 sec)