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HomeDatabaseMysql Tutorial关于mysql自增id的获取和重置_MySQL

 

mysql获取自增id的几种方法

使用max函数:select max(id) from tablename
优点:使用方便快捷。 缺点:获取的不是真正的自增id,是表中最大的Id,如果有删除数据的话,那么该值和自增id相差比较大。如果有连表数据,有可能导致数据错乱。 使用LAST_INSERT_ID函数:select LAST_INSERT_ID()
优点:获取到的是真正的自增id。 缺点:该函数是与table无关的,永远保留最新插入的自增列的id。如果多客户端操作,而且不能保证原子性,这个值可能会出现错误。所以需要在插入之后调用。 使用@@IDENTITY变量:select @@IDENTITY
此方法和LAST_INSERT_ID()功能差不多,优缺点也一致。需要在插入后调用。 使用mysql查询函数:SHOW TABLE STATUS;
优点:能够准确的查到自增id。而且可以在语句后面加上where语句或者like语句来过滤。 缺点:该语句返回的是一个记录集,不能单独的返回自增值。所以需要额外的操作来获取。 使用自定义查询方法:mysql表相关的信息是放在information_schema表里。所以我们参考 SHOW TABLE STATUS来构建查询语句。如:
select Auto_increment into autoId from information_schema.tables where Table_Schema = ‘dbname’ and table_name = ‘tablename’; 优点:高度自定义,能够准确的得到想得到的任何信息。 确定:不容易记住。呵呵。

mysql自增id的重置

使用truncate:truncate table;
说明:使用truncate会删除表的数据释放空间,并且重置字自增id,但不会删除表的定义。 用处:需要清空表的时候才能使用。 使用修改标识:dbcc checkident (‘table_name’ , reseed, new_reseed_value) ;
说明:new_reseed_value是设置的当前标识,下次插入则从new_reseed_value+1开始。如果new_reseed_value小于表中当前标识的值,有可能会产生重复key,会产生 2627 号错误信息。 用处:此方法不会清空已有数据。适用于大量删除后修改新的标识,但不能比当前表中最大值小。 使用修改表的方法:alter table tablename auto_increment=new
说明:此方法和上面的二方法类似。也不会清空数据,有可能会出现重复key的可能,所以此方法也只适用于清空表之后重置自增id或者大量删除后修改自增id。
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