约束条件Constraintcreatetalbestudent(idnumber(4)primarykey,主键约束：非空唯一约束namechar(20));复制表，但约束条件复制不过去。但可以这么做，先确定主键不重复，不为空，deletestudentwherename=‘tom’;删除ID重复记录altertablestudentaddconstraintstudent_id_pkprimarykey(id);student_id_pk是自定义的ALTERTABLEstuInfoADDCONSTRAINTPK_stuNoPRIMARYKEY(stuNo)主键约束ALTERTABLEstuInfoADDCONSTRAINTUQ_stuIDUNIQUE(stuID)唯一约束因为每人的身份证号全国唯一ALTERTABLEstuInfoADDCONSTRAINTDF_stuAddressDEFAULT('地址不详')FORstuAddress默认约束如果地址不填，默认为“地址不详ALTERTABLEstuInfoADDCONSTRAINTCK_stuAgeCHECK(stuAgeBETWEEN15AND40)检查约束要求年龄在15－40岁之间ALTERTABLEstuMarksADDCONSTRAINTFK_stuNoFOREIGNKEY(stuNo)REFERENCESstuInfo(stuNo)外键约束主表stuInfo和从表stuMarks建立关系，关联字段为stuNo删除约束：altertablestudentdropconstraint约束名;得先查约束名字selectconstraint_name,constraint_typefromuser_constraintswheretable_name=’STUDENT’;所以建议给约束都自己起名数据库中所有的表都是大写，得搜大写createtablestudent(idnumber(4)primarykey,namechar(20)notnull,emailchar(20)unique,sexchar(1)check(sexin(‘F’,’M’)));createtablestudent(idnumber(4),namechar(20)constraintstu_name_nnnotnull,emailchar(20),sexchar(1)，constraintstu_id_pkprimarykey(id),constraintstu_email_uqunique(email),constraintstu_sex_ckcheck(sexin(‘F’,’M’)));