如果您需要一个非主键自增字段,一个非常好的MySQL解决方案是使用相对较少知名的
last_insert_id(expr)
函数来创建任意序列。
如果将expr作为参数传递给LAST_INSERT_ID(),则该参数的值将被函数返回,并记忆为LAST_INSERT_ID()要返回的下一个值。这可以用于模拟序列...
(来自http://dev.mysql.com/doc/refman/5.1/en/information-functions.html#function_last-insert-id)
以下是一个示例,演示如何为每个帖子编号评论的次要序列:
CREATE TABLE `post` (
`id` INT(10) UNSIGNED NOT NULL,
`title` VARCHAR(100) NOT NULL,
`comment_sequence` INT(10) UNSIGNED NOT NULL DEFAULT '0',
PRIMARY KEY (`id`)
);
CREATE TABLE `comment` (
`id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
`post_id` INT(10) UNSIGNED NOT NULL,
`sequence` INT(10) UNSIGNED NOT NULL,
`content` TEXT NOT NULL,
PRIMARY KEY (`id`)
);
INSERT INTO post(id, title) VALUES(1, 'first post');
INSERT INTO post(id, title) VALUES(2, 'second post');
UPDATE post SET comment_sequence=Last_insert_id(comment_sequence+1) WHERE id=1;
INSERT INTO `comment`(post_id, sequence, content) VALUES(1, Last_insert_id(), 'blah');
UPDATE post SET comment_sequence=Last_insert_id(comment_sequence+1) WHERE id=1;
INSERT INTO `comment`(post_id, sequence, content) VALUES(1, Last_insert_id(), 'foo');
UPDATE post SET comment_sequence=Last_insert_id(comment_sequence+1) WHERE id=1;
INSERT INTO `comment`(post_id, sequence, content) VALUES(1, Last_insert_id(), 'bar');
UPDATE post SET comment_sequence=Last_insert_id(comment_sequence+1) WHERE id=2;
INSERT INTO `comment`(post_id, sequence, content) VALUES(2, Last_insert_id(), 'lorem');
UPDATE post SET comment_sequence=Last_insert_id(comment_sequence+1) WHERE id=2;
INSERT INTO `comment`(post_id, sequence, content) VALUES(2, Last_insert_id(), 'ipsum');
SELECT * FROM post;
SELECT * FROM comment;