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| CREATE TABLE `t2` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`a` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `idx_a` (`a`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
mysql> select * from t2;
+----+------+
| id | a |
+----+------+
| 1 | 2 |
| 3 | 4 |
| 6 | 5 |
| 8 | 5 |
| 10 | 5 |
| 13 | 11 |
+----+------+
|