添加字段

格式：alter table table_name add new_column data_type [interality_codition]

示例1ALTER TABLE student Add nationality varchar(20) --示例2 添加int类型的列,默认值为 0 alter table student add studentName int default 0 --示例3 添加int类型的列,默认值为0，主键 alter table student add studentId int primary key default 0 --示例4 判断student中是否存在name字段且删除字段 if exists(select * from syscolumns where id=object_id('student') and name='name') begin alter table student DROP COLUMN name end

更改字段

格式：alter table table_name alter column column_name
```
ALTER TABLE student ALTER COLUMN name VARCHAR(200)
```

删除字段

格式：alter table table_name drop column column_name
```
ALTER TABLE student DROP COLUMN nationality;
```

查看字段约束

格式： select * from information_schema.constraint_column_usage where TABLE_NAME = table_name

SELECT TABLE_NAME,COLUMN_NAME,CONSTRAINT_NAME FROM information_schema.CONSTRAINT_COLUMN_USAGE WHERE TABLE_NAME = 'student'

查看字段缺省约束表达式（即默认值等）

格式：select * from information_schema.columns where TABLE_NAME = table_name
```
SELECT TABLE_NAME, COLUMN_NAME, COLUMN_DEFAULT FROM information_schema.COLUMNS WHERE TABLE_NAME='student'
```

查看字段缺省约束名

格式：select name from sysobjects where object_id(table_name)=parent_obj and xtype=’D’
```
select name from sysobjectswhere object_id('表?名?')=parent_obj and xtype='D'
```

删除字段约束

格式：alter table tablename drop constraint constraintname
```
ALTER TABLE student DROP CONSTRAINT PK__student__2F36BC5B772B9A0B
```

添加字段约束

格式：alter table tablename add constraint constraintname primary key (column_name)

--示例1ALTER TABLE stuInfo ADD CONSTRAINT PK_stuNo PRIMARY KEY (stuNo) --示例2 添加主键约束（Primary Key） -- 存在主键约束PK_stuNO，则删除 IF EXISTS(SELECT * FROM sysobjects WHERE name='PK_stuNo' and xtype='PK') Alter TABLE stuInfo Drop Constraint PK_stuNo Go -- 重新添加主键约束PK_stuNO ALTER TABLE stuInfo ADD CONSTRAINT PK_stuNo PRIMARY KEY (stuNo) Go --示例3 添加 唯一UQ约束（Unique Constraint） -- 存在唯一约束UQ_stuNO，则删除 IF EXISTS(SELECT * FROM sysobjects WHERE name='UQ_stuID' and xtype='UQ') Alter TABLE stuInfo Drop Constraint UQ_stuID Go -- 重新添加唯一约束UQ_stuID ALTER TABLE stuInfo ADD CONSTRAINT UQ_stuID UNIQUE (stuID) --示例4 添加默认DF约束（Default Constraint） -- 存在默认约束UQ_stuNO，则删除 IF EXISTS(SELECT * FROM sysobjects WHERE name='DF_stuAddress' and xtype='D') Alter TABLE stuInfo Drop Constraint DF_stuAddress Go -- 重新添加默认约束DF_stuAddress ALTER TABLE stuInfo ADD CONSTRAINT DF_stuAddress DEFAULT ('地址不详') FOR stuAddress --示例5 检查CK约束（Check Constraint） -- 存在检查约束UQ_stuNO，则删除 IF EXISTS(SELECT * FROM sysobjects WHERE name='CK_stuAge' and xtype='C') Alter TABLE stuInfo Drop Cons

转载于:https://www.cnblogs.com/lichanghui/p/4845870.html

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