我想在一个表中增加一个不能为空的字段,为什么不行呢?

--drop table index_tb

create table index_tb(id int constraint ck_id default 0,name varchar(20),xb varchar(2))

--查看表

exec sp_helpindex orders--索引

exec   sp_helpconstraint  index_tb --约束

exec   sp_fkeys   'dbo.Person'--外键

--键不唯一,非聚集索引

create index id_index on index_tb(id)

--删除索引

drop index index_tb.id_index

--键聚集索引

create clustered index id_index on index_tb(id) 

--删除索引

drop index index_tb.id_index

--创建check约束

alter table index_tb add constraint ck_index CHECK (name like '[0-9][0-9][0-9][0-9][0-9]')

alter table index_tb add constraint ck2_index check(xb in ('MM','GG'))

--删除check约束

alter table index_tb drop constraint ck2_index

--禁止表约束

alter table index_tb nocheck constraint ck_index

--恢复表约束

alter table index_tb check constraint ck_index

--禁止和恢复所有check-all关键字

select 'alter table '+name+' nocheck or check constraint all' from sysobjects where type='U'

--添加新默认值和约束

ALTER TABLE index_tb ADD CONSTRAINT de_name  DEFAULT ('11111') FOR name

--删除表约束

alter table index_tb drop constraint de_name

-- 字段  UNIQUE NOT NULL = PRIMARY KEY 差不多

--创建unique

alter table index_tb add constraint uniqueindex unique(id)

 --删除unique

alter table index_tb drop constraint uniqueindex



insert into index_tb(name,xb) select '12345','mm'



select * from index_tb



delete index_tb