--SchoolDB字段创建添加约束以及主键CREATEDATABASESchoolDB;USESchoolDB;CREATETABLEGrade(gradeIdINT(4)NOTNULL,gradeNameVARCHAR(50)NOTNULL,PRIMARYKEY(gradeId));CREATETABLESubject(subjectIdINT(4)NOTNULL,subjectNameVARCHAR(20),classH

--schooldb字段创建添加约束以及主键创建数据库和表（之前已经创建的部分）CREATEDATABASESchoolDB;USESchoolDB;CREATETABLEGrade(gradeIdINT(4)NOTNULL,gradeNameVARCHAR(50)NOTNULL,PRIMARYKEY(gradeId));CREATETABLESubject(subjectIdINT(4)NOTNULL,