维护SQL Server数据库的一些常用SQL
REIGN KEY (StudentID) REFERENCES Students (StudentID) ON DELETE CASCADE
23.怎样删除表的一个外键？
ALTER TABLE Scores DROP CONSTRAINT FK_Scores_Students
24.怎样给字段加上CHECK约束？
ALTER TABLE Students ADD CONSTRAINT CK_Students CHECK (Age > 0)
25.怎样去掉字段上的CHECK约束？
ALTER TABLE Students DROP CONSTRAINT CK_Students
26.怎样给字段设置默认值？
ALTER TABLE Students ADD CONSTRAINT DF_Students_Age DEFAULT (18) FOR Age
27.怎样移去字段的默认值？
ALTER TABLE Students DROP CONSTRAINT DF_Students_Age
28.修改字段的类型及非空约束
ALTER TABLE Students ALTER COLUMN Age char (10) null
ALTER TABLE Students ALTER COLUMN Age int not null