title: PRIMARY KEY for MERGE engine don't working?
body: <p>My code : </p>
  
  <pre><code>mysql&gt; CREATE TABLE super (id int PRIMARY KEY NOT NULL AUTO_INCREMENT);
  mysql&gt; CREATE TABLE super2 (id int PRIMARY KEY NOT NULL AUTO_INCREMENT);
  
  mysql&gt; INSERT INTO super VALUES(1),(2);
  mysql&gt; INSERT INTO super2 VALUES(1),(3);
  
  mysql&gt; ALTER TABLE super ENGINE='MYIsam';
  mysql&gt; ALTER TABLE super2 ENGINE='MYIsam' 
  </code></pre>
  
  <p>Then I created MERGE Engine table :</p>
  
  <pre><code>mysql&gt; CREATE TABLE super_merge (id int PRIMARY KEY NOT NULL AUTO_INCREMENT) E
  INE='MERGE' UNION=(super,super2);
  Query OK, 0 rows affected (0.00 sec)
  
  mysql&gt; SELECT * FROM super_merge;
  +----+
  | id |
  +----+
  |  1 |
  |  1 |
  |  2 |
  |  3 |
  +----+ 
  </code></pre>
  
  <p>Will not the rule of uniqueness?Maybe this is normal .</p>
  
  <p>Why, then, to create a structure of table fields if it does not work? </p>
  
  <p>When I truncate tables super,super2 then table super_merge is empty too!</p>
  
  <pre><code>TRUNCATE TABLE super2;
  mysql&gt; SELECT * FROM super_merge;
  Empty set (0.00 sec)
  
  mysql&gt; INSERT INTO super VALUES(1),(2);
  mysql&gt; INSERT INTO super2 VALUES(1),(3);
  
  mysql&gt; SELECT * FROM super_merge;
  +----+
  | id |
  +----+
  |  1 |
  |  1 |
  |  2 |
  |  3 |
  </code></pre>