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20.9. 重新整理AUTO_INCREMENT字段

AUTO_INCREMENT 并非按照我们意愿,顺序排列,经常会跳过一些数字,例如当插入失败的时候,再次插入会使用新的值。有时会造成浪费,我们可以使用下面SQL重新编排AUTO_INCREMENT序列。

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

使用max()查看最大值,然后使用 alter修改起始位置。

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

注意外键,需要 ON UPDATE CASCADE 支持,否则无法更新。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 '手机号码',
	`email` VARBINARY(50) NULL DEFAULT NULL COMMENT '电子邮件',
	`mobile_digest` VARCHAR(32) NULL DEFAULT NULL COMMENT '摘要',
	`email_digest` VARCHAR(32) NULL DEFAULT NULL COMMENT '邮件摘要',
	`birthday` DATE NULL DEFAULT NULL COMMENT '生日',
	`description` VARCHAR(255) NULL DEFAULT NULL COMMENT '备注描述',
	`status` ENUM('Subscription','Unsubscribe') NOT NULL DEFAULT 'Subscription' COMMENT '订阅状态',
	`ctime` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '创建时间',
	`mtime` TIMESTAMP NULL DEFAULT NULL ON UPDATE CURRENT_TIMESTAMP COMMENT '修改时间',
	PRIMARY KEY (`id`),
	UNIQUE INDEX `digest` (`mobile_digest`, `email_digest`)
)
COMMENT='会员手机短信与电子邮件映射表'
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='短信分组'
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;
		
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