sql server中有五种约束
使用sql语句创建和删除约束
主建约束:(primary key constraint);外建约束:(foreign key constraint);唯一约束:(unique constraint);检查约束:(check constraint);默认约束:(default constraint);**********************************************************************
添加约束:alter table tablenameadd constraint pk_colname primary key(colname)主建约束alter table tablenameadd constraint uq_colname unique (colname)唯一约束alter table tablenameadd constraint df_colname default('地址不详')for colname 默认约束alter table tablenameadd constraint ck_colname check(colname between 12 and 15)检查约束alter table tablenameadd constraint fk_colname foreign key(colname)references tablename(colname)外建约束删除约束:alter table tablenamedrop constraint 约束名