一、identity的基本用法
1.含义
(资料图)
identity表示该字段的值会自动更新,不需要我们维护,通常情况下我们不可以直接给identity修饰的字符赋值,否则编译时会报错
2.语法
列名 数据类型 约束 identity(m,n)
m表示的是初始值,n表示的是每次自动增加的值
如果m和n的值都没有指定,默认为(1,1)
要么同时指定m和n的值,要么m和n都不指定,不能只写其中一个值,不然会出错
3.实例演示
不指定m和n的值
create table student1 ( sid int primary key identity, sname nchar(8) not null, ssex nchar(1) ) insert into student1(sname,ssex) values ("张三","男"); insert into student1 values ("李四","女");--可以省略列名 insert into student1 values ("王五","女");
指定m和n的值
create table student2 ( sid int primary key identity(20,5), sname nchar(8) not null, ssex nchar(1) ) insert into student2(sname,ssex) values ("张三","男"); insert into student2 values ("李四","女");--可以省略列名 insert into student2 values ("王五","女");
4.删除一条记录接着插入
把sid为2的记录删除,继续插入,新插入的记录的sid不是2,而是3
create table student3 ( sid int primary key identity, sname nchar(8) not null, ssex nchar(1) ) insert into student3(sname,ssex) values ("张三","男"); insert into student3 values ("李四","女"); delete from student3 where sid=2;--把sid为2的记录删除 insert into student3 values ("王五","女");
二、重新设置identity的值
1.语法
dbcc checkident(表名,reseed,n);
n+1表示的是表中identity字段的初始值(n的值可以为0)
也就是说:如果插入的是id为2的记录,则n的值是1
2.实例演示
create table student4(sid int primary key identity,sname nchar(8) not null,ssex nchar(1))insert into student4(sname,ssex) values ("张三","男");insert into student4 values ("李四","女");delete from student4 where sid=2;--把sid为2的记录删除dbcc checkident("student4",reseed,1);--把student4表中identity字段的初始值重新设置为1insert into student4 values ("王五","女");
三、向identity字段插入数据
1.语法
set identity_insert 表名 on; insert into 表名(列名1,列名2,列名3,列名4) values (数据1,数据2,数据3,数据4); set identity_insert 表名 off;
注意:插入数据时必须得指定identity修饰的字段的名字
2.实例演示
create table student5(sid int primary key identity(20,5),sname nchar(8) not null,ssex nchar(1))insert into student5(sname,ssex) values ("张三","男");insert into student5 values ("李四","女");insert into student5 values ("王五","女");set identity_insert student5 on;/*insert into student5 values ("黑六","男");--errorinsert into student5 values (21,"黑六","男");--error*/insert into student5(sid,sname,ssex) values (21,"黑六","男");set identity_insert student5 off;/*insert into student5 values (22,"赵七","女");--errorinsert into student5(sid,sname,ssex) values (22,"赵七","女");--error*/insert into student5 values ("赵七","女");