database - Oracle SQL Developer - Adding Constraints on Foreign Key -

I'm new to Oracle SQL and I'm having a hard time adding an obstacle. I am trying to add obstacles in my table to implement specific business rules which only allow students to take up to 4 courses and a maximum of 25 students per class.

Please tell me what additional information I need to help me answer this question. I am in a loss ...

  Make Table Grade (STU_ID int not gross enabled, CRSE_ID CHAR (9) No, enable enabled, STU_CRSE_GRADE VARCHAR2 (20) Check (STU_CRSE_GRADE = 'A' Or STU_CRSE_GRADE = 'B' or STU_CRSE_GRADE = 'C' or STU_CRSE_GRADE = 'D' or STU_CRSE_GRADE = 'F'), CONSTRAINT GRADES_PK Primary key (STU_ID, CRSE_ID), Obligation fk_Grades Foreign Key (Stu_ID) reference student, obligation fk_Grades_Crse_ID foreign key Crse_ID Reference Reference Course);   
 No problem! See tables below:  
  Make table students (Stu_ID int constraint pk_Stu_ID Primary key, stu_name VARCHAR (255) No, No. Stu_Add varchar (255), Stumaj CHAR (6)); Create a Table Instructor (Instr_ID char (3) Barrier pk_Instr_ID Primary key, Instr_Name VARCHAR (255) faucet, Insta_office formatting (8)); Create table course (CRSEID four (9) barrier pk_Crse_ID primary key, CRCITETRRARAr (255) No, student name: Lai Xia Instra_ID CRAR (3) No faucet, hindered fk_Courses_Instr_ID foreign key (Instr_ID reference instructor);    
 
 
 foreign key  alone  only one-many relationships If you want to limit the "many" parts to a specific number, then you will either need it:  
 
 Do this in the application code or trigger.  
 Or again redefine FK in "Help" to get the remaining target database. It's easy to implement (1), but it's easy to do the wrong: You have to be careful about the  Concurrent  environment to avoid race conditions. Arrange locking for:  
 
 Let's assume that two concurrent transactions are trying to add the same student to a different course.  
 The first transaction currently counters the courses associated with the student and finds that 3 of them are all good and good.  
 The second transaction is the same and only sees 3 courses (because the first transaction is not committed yet).  
 So both transactions think that they are more than the 'allowed number' and proceed happily, while adding  both  to their student-curriculum connection.  
 Last result: Student is affiliated with 5 courses, violates the rule that it can be added only for 4.   
 To avoid this, you will need to  serialize  these actions, possibly updating and locating the student via  SELECT ... .  
 
 By changing the key design (2) can be implemented, and then one key can limit the values. For example, to apply it, a student can take up to 4 courses, can be done this way:  
  Make Table Student (STUDENT_ID INT Primary Key); Create Table Course (Corio_ID INT Primary Key); Make Table STUDENT_COURSE (STUDENT_ID INT Reference Student, Course IID INT Reference Course, Course INO INT No Faucet Check (Corosive_NA in (1, 2, 3, 4)), Primary Key (STUDENT_ID, COURSE_ID), Unique (STUDENT_ID, COURSE_NO) ;   
 A combination of checks and unique obstacles means that DBMS itself will refuse to connect the student to more than 4 courses.  
 This will be successful by:  
  STUDENT_COURSE value in INSERT (11, 111, 1); INSERT (11, 222, 2) in STUDENT_COURSE VALUES; INSERT (11, 333, 3) in STUDENT_COURSE values; INSERT (11, 444, 4) in STUDENT_COURSE values;   
 But this will not be obvious (violation of check barrier):  
  STUDENT_COURSE VALUES INSERT (11, 555, 5); BTW, when the student is already connected to some courses and you want to find the remaining free "slot", you can do it like this:  
  Select from NEW_NO SELECT COURSE_NO + 1 NEW_NO, LEAD (COURSE_NO) (by course B_NO) STUDENT_COURSE NEXT_NO WHERE STUDENT_ID = 11) Where NEW_NO & lt; & Gt; NEXT_NO or NEXT_NO is Null;