AUTO_INCREMENT 並非按照我們意願，順序排列，經常會跳過一些數字，例如當插入失敗的時候，再次插入會使用新的值。有時會造成浪費，我們可以使用下面SQL重新編排AUTO_INCREMENT序列。

SET @newid=0;
UPDATE mytable SET id = (SELECT @newid:=@newid+ 1);
		

使用max()查看最大值，然後使用 alter修改起始位置。

select max(id) from mytable;
ALTER TABLE mytable AUTO_INCREMENT = 1000;		
		

注意外鍵，需要 ON UPDATE CASCADE 支持，否則無法更新。CONSTRAINT `FK_group_has_contact_contact` FOREIGN KEY (`contact_id`) REFERENCES `contact` (`id`) ON UPDATE CASCADE ON DELETE CASCADE,

CREATE TABLE `contact` (
	`id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT COMMENT '唯一ID',
	`name` VARCHAR(50) NOT NULL COMMENT '姓名',
	`mobile` VARBINARY(32) NULL DEFAULT NULL COMMENT '手機號碼',
	`email` VARBINARY(50) NULL DEFAULT NULL COMMENT '電子郵件',
	`mobile_digest` VARCHAR(32) NULL DEFAULT NULL COMMENT '摘要',
	`email_digest` VARCHAR(32) NULL DEFAULT NULL COMMENT '郵件摘要',
	`birthday` DATE NULL DEFAULT NULL COMMENT '生日',
	`description` VARCHAR(255) NULL DEFAULT NULL COMMENT '備註描述',
	`status` ENUM('Subscription','Unsubscribe') NOT NULL DEFAULT 'Subscription' COMMENT '訂閲狀態',
	`ctime` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '創建時間',
	`mtime` TIMESTAMP NULL DEFAULT NULL ON UPDATE CURRENT_TIMESTAMP COMMENT '修改時間',
	PRIMARY KEY (`id`),
	UNIQUE INDEX `digest` (`mobile_digest`, `email_digest`)
)
COMMENT='會員手機短信與電子郵件映射表'
COLLATE='utf8_general_ci'
ENGINE=InnoDB
AUTO_INCREMENT=43642;

CREATE TABLE `group` (
	`id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
	`name` VARCHAR(50) NOT NULL,
	`description` VARCHAR(512) NOT NULL,
	`ctime` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
	PRIMARY KEY (`id`),
	UNIQUE INDEX `name` (`name`)
)
COMMENT='短信分組'
COLLATE='utf8_general_ci'
ENGINE=InnoDB
AUTO_INCREMENT=8;

CREATE TABLE `group_has_contact` (
	`id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
	`group_id` INT(10) UNSIGNED NOT NULL,
	`contact_id` INT(10) UNSIGNED NOT NULL,
	`ctime` TIMESTAMP NULL DEFAULT CURRENT_TIMESTAMP,
	PRIMARY KEY (`id`),
	UNIQUE INDEX `group_contact` (`group_id`, `contact_id`),
	INDEX `FK_group_has_contact_contact` (`contact_id`),
	CONSTRAINT `FK_group_has_contact_contact` FOREIGN KEY (`contact_id`) REFERENCES `contact` (`id`) ON UPDATE CASCADE ON DELETE CASCADE,
	CONSTRAINT `FK_group_has_contact_group` FOREIGN KEY (`group_id`) REFERENCES `group` (`id`) ON UPDATE CASCADE ON DELETE CASCADE
)
COMMENT='N:M'
COLLATE='utf8_general_ci'
ENGINE=InnoDB
AUTO_INCREMENT=55764;