How to write stored procedure in this case? - sql

Question

Database: training
Task: write a stored procedure for transferring a student from one group to another, while checking whether both groups have the same number of disciplines. This is, for example, in one group the number of students was 28, and in the other 20. So that after the procedure, the number was 27 and 21. The code for creating the database tables:...
CREATE DATABASE education
GO
USE education
CREATE TABLE control_type
(
control_type_id INT IDENTITY(1,1) PRIMARY KEY,
control_type_name VARCHAR(20) NOT NULL
);
CREATE TABLE category
(
category_id INT IDENTITY(1,1) PRIMARY KEY,
category_name VARCHAR(20) NOT NULL
);
CREATE TABLE subjects
(
subject_id INT IDENTITY(1,1) PRIMARY KEY,
subject_name VARCHAR(20) NOT NULL,
count_hours INT NOT NULL,
category_id INT NOT NULL
REFERENCES category (category_id)
ON DELETE CASCADE
);
CREATE TABLE groups
(
group_id INT IDENTITY(1,1) PRIMARY KEY,
group_name VARCHAR(20) NOT NULL,
course INT NOT NULL DEFAULT '1',
count_students INT NOT NULL,
count_hours INT NOT NULL
);
CREATE TABLE [session]
(
session_id INT IDENTITY(1,1) PRIMARY KEY,
name_last_prof VARCHAR(20) NOT NULL,
name_1st_prof VARCHAR(20) NOT NULL,
name_2nd_prof VARCHAR(20) NOT NULL,
session_date DATE NOT NULL,
control_type_id INT NOT NULL
REFERENCES control_type (control_type_id)
ON DELETE CASCADE,
group_id INT NOT NULL
REFERENCES groups (group_id)
ON DELETE CASCADE,
subject_id INT NOT NULL
REFERENCES subjects (subject_id)
ON DELETE CASCADE
);
CREATE INDEX idx_group ON groups (count_students);
CREATE INDEX idx_session ON [session] (name_last_prof);
CREATE INDEX idx_subject ON subjects (count_hours);
CREATE INDEX idx_category ON category (category_name);
CREATE INDEX idx_control_type ON control_type (control_type_name);
SET dateformat dmy;
INSERT INTO category VALUES ('�����������');
INSERT INTO category VALUES ('����*������');
INSERT INTO category VALUES ('�����������');
INSERT INTO category VALUES ('��������-���������');
INSERT INTO category VALUES ('���������');
INSERT INTO category VALUES ('��������');
INSERT INTO category VALUES ('������');
INSERT INTO category VALUES ('�������');
INSERT INTO category VALUES ('��������');
INSERT INTO category VALUES ('������');
INSERT INTO control_type VALUES ('����');
INSERT INTO control_type VALUES ('�����');
INSERT INTO subjects VALUES ('��������� ����',80,1);
INSERT INTO subjects VALUES ('��������� ��',75,1);
INSERT INTO subjects VALUES ('������ �������',90,3);
INSERT INTO subjects VALUES ('��������� ����������',100,3);
INSERT INTO subjects VALUES ('������������ �����',120,3);
INSERT INTO subjects VALUES ('���������',110,6);
INSERT INTO subjects VALUES ('��������',100,6);
INSERT INTO subjects VALUES ('�����������',120,6);
INSERT INTO subjects VALUES ('��������',80,6);
INSERT INTO subjects VALUES ('������',100,7);
INSERT INTO subjects VALUES ('������ ������',80,5);
INSERT INTO groups VALUES ('���-01-1',1,20,500);
INSERT INTO groups VALUES ('���-01-2',1,25,520);
INSERT INTO groups VALUES ('ϲ-171',3,23,550);
INSERT INTO groups VALUES ('ϲ-172',3,22,540);
INSERT INTO groups VALUES ('ʲ-181',2,30,490);
INSERT INTO groups VALUES ('ʲ-182',2,29,500);
INSERT INTO groups VALUES ('��-161',4,28,480);
INSERT INTO groups VALUES ('��-162',4,24,510);
INSERT INTO groups VALUES ('��-171',3,26,500);
INSERT INTO groups VALUES ('��-172',3,23,520);
INSERT INTO [session] VALUES ('�����','���','�������','22.06.2022',1,1,3);
INSERT INTO [session] VALUES ('����','����','�������','17.01.22',2,1,4);
INSERT INTO [session] VALUES ('������','³����','�������������','20.06.2022',2,2,3);
INSERT INTO [session] VALUES ('��������','���������','�������������','15.06.2022',2,2,2);
INSERT INTO [session] VALUES ('�����','�����','�������','20.01.22',1,5,8);
INSERT INTO [session] VALUES ('������','���������','����������','18.06.2022',2,6,7);
INSERT INTO [session] VALUES ('�����','�����','³��������','23.01.22',2,3,6);
INSERT INTO [session] VALUES ('�����������','������','��������','16.01.22',2,1,10);
INSERT INTO [session] VALUES ('������','������','����������','13.06.2022',1,2,2);
INSERT INTO [session] VALUES ('������','�����','��������','15.01.22',2,2,4);
I don't understand the logic of writing SQL queries in this case...