废话不多说，直接上代码

create database mydb
use mydb
go
create table account(
  id varchar(16),
  name varchar(16),
  balance float
)
go
select * from account

insert into account(id, name, balance) values('620101', 'liyong', 300)
insert into account(id, name, balance) values('620106', 'mali', 400)
--insert into account(id, name, balance) values('620009', 'chenying', 800)
insert into account(id, name, balance) values('646009', 'chenying', 800)
--delete from account where id = '620009'
go
update account set balance = balance - 1000 where id = '620101'
update account set balance = balance + 1000 where id = '620106'
--消息 547，级别 16，状态 0，第 1 行
--UPDATE 语句与 CHECK 约束"CK_Blance"冲突。该冲突发生于数据库"mydb"，表"dbo.account", column 'balance'。
--语句已终止。

go
--alter table account
--alter COlumn balance int
go
alter table account
add constraint CK_Blance check(balance >= 0)
go
alter table account
drop constraint CK_Blance
--定一个事务
--从liyong扣钱往mali加钱
begin transaction
update account set balance = balance - 1000 where id = '620101'
if((select balance output from account where id = '620101') < 0)
begin
PRINT('余额不足！');
ROLLBACK;
end
else
begin
  update account set balance = balance + 1000 where id = '620106'
  commit;
  PRINT('转账成功！');
end
go
sp_help
--备份设备
sp_addumpdevice 'disk', 'xk_bak' ,'d:\xk_bak'
--备份数据库
backup database mydb
to xk_bak
--还原数据库
restore database mydb from disk = 'd:\xk_bak'
with replace; --覆盖