مالخطاء في هذا الكود؟

drop table substance;
drop table Instructor;
drop table research;
drop table department;
drop table activity;
go
CREATE TABLE  activity (
act_name  varchar (15) NOT NULL ,
act_no    nvarchar (50) NOT NULL ,
act_date  datetime NULL, 
constraint pk_act primary key (act_no));
go

CREATE TABLE  department (
dep_no    varchar (50) NOT NULL ,
dep_name  varchar (15) NOT NULL, 
constraint pk_dep primary key (dep_no));
GO

CREATE TABLE research (
res_name  varchar (15) NOT NULL ,
res_no    nvarchar (50) NOT NULL ,
res_date  varchar (10) NULL ,
constraint pk_res primary key (res_no));
go

CREATE TABLE student (
dep_no    varchar (50) NULL ,
act_no    nvarchar (50) NULL ,
Stu_no    varchar (10) NOT NULL ,
Stu_name  varchar (15) NOT NULL ,
res_no    nvarchar (50) NULL ,
e_stu     datetime NULL ,
Address  varchar (15) NOT NULL ,
constraint pk_stu primary key (stu_no),
constraint fk_stu_dep foreign key (dep_no)
references department (dep_no)
constraint fk_stu_act foreign key (act_no)
references activity (act_no)
constraint fk_stu_res foreign key (res_no)
references research (res_no));
GO

CREATE TABLE exam (
e_date  varchar (10) NULL ,
e_name  varchar (15) NOT NULL ,
e_no    varchar (10) NOT NULL ,
Stu_no  varchar (10) NOT NULL ,
constraint pk_e primary key (e_no),
constraint fk_e_stu foreign key (stu_no)
references student(stu_no));
go

CREATE TABLE Instructor (
inst_no     int  NOT NULL ,
inst_name   varchar (15) NOT NULL ,
dep_no      varchar (50) NULL ,
nationality  varchar (15) NOT NULL,
constraint pk_inst primary key (inst_no),
constraint fk_inst_dep foreign key (dep_no)
references department(dep_no));
GO

CREATE TABLE substance (
sub_no      varchar (10) NOT NULL ,
class_name  varchar (15) NOT NULL ,
quar        char (15) NULL ,
inst_no     int NOT NULL, 
constraint pk_sub primary key (sub_no),
constraint fk_sub_inst foreign key (inst_no)
references Instructor(inst_no));

تظهر رسالة الخطاء التالية

Server: Msg 156, Level 15, State 1, Line 12
Incorrect syntax near the keyword 'constraint'.
Server: Msg 1767, Level 16, State 1, Line 1
Foreign key 'fk_e_stu' references invalid table 'student'.
Server: Msg 1750, Level 16, State 1, Line 1
Could not create constraint. See previous errors.

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