diff --git a/excercise1.sql b/excercise1.sql
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+/*Database
+
+Mỗi bài tập sẽ lưu trên file sol-{number}.sql
+
+1. Giải bài tập trên leetcode: 
+
+https://leetcode.com/problems/capital-gainloss/description/ (gợi ý: sử dụng CASE)*/
+--------------------------------------------------------------------------------------
+select new.stock_name, sum(newprice) as capital_gain_loss from (
+    select old.stock_name, 
+    CASE
+    WHEN operation = 'Buy' THEN -price
+    ELSE price
+    END AS newprice
+    from Stocks as old
+) as new group by stock_name;
+--------------------------------------------------------------------------------------
+/*https://leetcode.com/problems/count-salary-categories/ (ngoài các cách trên leetcode, hãy nghĩ cách để giúp câu query này nhanh hơn, kể cả thay đổi cấu trúc bản)*/
+--------------------------------------------------------------------------------------
+select 
+new.category, count(new.category)-1 as accounts_count
+from (
+    select
+    CASE
+    WHEN old.income > 50000 THEN 'High Salary'
+    WHEN old.income < 20000 THEN 'Low Salary'
+    ELSE 'Average Salary'
+    END AS category
+    from Accounts as old
+    UNION ALL
+    SELECT 'High Salary' as category
+    UNION ALL
+    SELECT 'Low Salary' as category
+    UNION ALL
+    SELECT 'Average Salary' as category
+) as new group by new.category;
+--------------------------------------------------------------------------------------
+/*2. Bạn hãy viết một script để tạo các bản cho hệ thống với cấu trúc ở dưới
+
+![img.png](img.png)
+
+hệ thống bao gồm:
+
+- class
+- professor: quan hệ one-many với class
+- student: quan hệ many-many với class
+- course: quan hệ one-many với class
+- room: quan hệ one-one với class*/
+--------------------------------------------------------------------------------------
+SET @OLD_UNIQUE_CHECKS=@@UNIQUE_CHECKS, UNIQUE_CHECKS=0;
+SET @OLD_FOREIGN_KEY_CHECKS=@@FOREIGN_KEY_CHECKS, FOREIGN_KEY_CHECKS=0;
+SET @OLD_SQL_MODE=@@SQL_MODE, SQL_MODE='TRADITIONAL,ALLOW_INVALID_DATES';
+--
+DROP DATABASE IF EXISTS `SCHOOLDB`;
+CREATE DATABASE IF NOT EXISTS `SCHOOLDB`;
+USE `SCHOOLDB`;
+DROP SCHEMA IF EXISTS `COLLEGE`;
+CREATE SCHEMA `COLLEGE` DEFAULT CHARACTER SET utf8;
+USE `COLLEGE`;
+--
+DROP TABLE IF EXISTS `COLLEGE`.`COURSE`;
+
+CREATE TABLE IF NOT EXISTS `COLLEGE`.`COURSE` (
+ `COURSE_ID` INT NOT NULL UNIQUE AUTO_INCREMENT,
+ `COURSE_NAME` VARCHAR(255) NOT NULL,
+ PRIMARY KEY (`COURSE_ID`));
+--
+DROP TABLE IF EXISTS `COLLEGE`.`PROFESSOR`;
+
+CREATE TABLE IF NOT EXISTS `COLLEGE`.`PROFESSOR` (
+ `PROF_ID` INT NOT NULL UNIQUE AUTO_INCREMENT,
+ `PROF_LNAME` VARCHAR(50) NOT NULL,
+ `PROF_FNAME` VARCHAR(50) NOT NULL,
+ PRIMARY KEY (`PROF_ID`));
+--
+DROP TABLE IF EXISTS `COLLEGE`.`STUDENT`;
+
+CREATE TABLE IF NOT EXISTS `COLLEGE`.`STUDENT` (
+ `STUD_ID` INT NOT NULL UNIQUE AUTO_INCREMENT,
+ `STUD_FNAME` VARCHAR(50) NOT NULL,
+ `STUD_LNAME` VARCHAR(50) NOT NULL,
+ `STUD_STREET` VARCHAR(255) NOT NULL,
+ `STUD_CITY` VARCHAR(50) NOT NULL,
+ `STUD_ZIP` VARCHAR(10) NOT NULL,
+ PRIMARY KEY (`STUD_ID`));
+--
+DROP TABLE IF EXISTS `COLLEGE`.`CLASS`;
+
+CREATE TABLE IF NOT EXISTS `COLLEGE`.`CLASS` (
+ `CLASS_ID` INT NOT NULL UNIQUE AUTO_INCREMENT,
+ `CLASS_NAME` VARCHAR(255) NOT NULL,
+ `PROF_ID` INT NOT NULL,
+ `COURSE_ID` INT NOT NULL,
+ `ROOM_ID` INT NOT NULL,
+ PRIMARY KEY (`CLASS_ID`),
+ INDEX `PROF_ID_IDX` (`PROF_ID` ASC),
+ INDEX `COURSE_ID_IDX` (`COURSE_ID` ASC),
+ INDEX `ROOM_ID_IDX` (`ROOM_ID` ASC),
+ CONSTRAINT `PROF_CLASS_FK` FOREIGN KEY (`PROF_ID`) REFERENCES `COLLEGE`.`PROFESSOR` (`PROF_ID`)
+ ON DELETE RESTRICT
+ ON UPDATE RESTRICT,
+ CONSTRAINT `COURSE_CLASS_FK` FOREIGN KEY (`COURSE_ID`) REFERENCES `COLLEGE`.`COURSE` (`COURSE_ID`)
+ ON DELETE RESTRICT
+ ON UPDATE RESTRICT,
+ CONSTRAINT `ROOM_CLASS_FK` FOREIGN KEY (`ROOM_ID`) REFERENCES `COLLEGE`.`ROOM` (`ROOM_ID`)
+ ON DELETE RESTRICT
+ ON UPDATE RESTRICT);
+--
+ DROP TABLE IF EXISTS `COLLEGE`.`ENROLL`;
+
+CREATE TABLE IF NOT EXISTS `COLLEGE`.`ENROLL` (
+ `STUD_ID` INT NOT NULL,
+ `CLASS_ID` INT NOT NULL,
+ `GRADE` VARCHAR(3) NOT NULL,
+ CONSTRAINT ENROLL_PK PRIMARY KEY (`STUD_ID`, `CLASS_ID`),
+ CONSTRAINT `STUD_ENROLL_FK` FOREIGN KEY (`STUD_ID`) REFERENCES `COLLEGE`.`STUDENT` (`STUD_ID`)
+ ON DELETE RESTRICT
+ ON UPDATE RESTRICT,
+ CONSTRAINT `CLASS_ENROLL_FK` FOREIGN KEY (`CLASS_ID`) REFERENCES `COLLEGE`.`CLASS` (`CLASS_ID`)
+ ON DELETE RESTRICT
+ ON UPDATE RESTRICT);
+ --
+DROP TABLE IF EXISTS `COLLEGE`.`ROOM`;
+
+CREATE TABLE IF NOT EXISTS `COLLEGE`.`ROOM` (
+ `ROOM_ID` INT NOT NULL UNIQUE AUTO_INCREMENT,
+ `ROOM_LOC` VARCHAR(50) NOT NULL,
+ `ROOM_CAP` VARCHAR(50) NOT NULL,
+ `CLASS_ID` INT NOT NULL,
+ PRIMARY KEY (`ROOM_ID`),
+ CONSTRAINT `CLASS_ROOM_FK` FOREIGN KEY (`CLASS_ID`) REFERENCES `COLLEGE`.`CLASS` (`CLASS_ID`)
+ ON DELETE RESTRICT
+ ON UPDATE RESTRICT);
+--
+SET SQL_MODE=@OLD_SQL_MODE;
+SET FOREIGN_KEY_CHECKS=@OLD_FOREIGN_KEY_CHECKS;
+SET UNIQUE_CHECKS=@OLD_UNIQUE_CHECKS;
+--------------------------------------------------------------------------------------
+/*3. Hãy viết câu query để tìm:
+a những cặp student-professor có dạy học nhau và số lớp mà họ có liên quan
+b những course (distinct) mà 1 professor cụ thể đang dạy
+c những course (distinct) mà 1 student cụ thể đang học
+d điểm số là A, B, C, D, E, F tương đương với 10, 8, 6, 4, 2, 0
+e điểm số trung bình của 1 học sinh cụ thể (quy ra lại theo chữ cái, và xếp loại học lực (weak nếu avg < 5, average nếu >=5 < 8, good nếu >=8 )
+f điểm số trung bình của các class (quy ra lại theo chữ cái)
+g điểm số trung bình của các course (quy ra lại theo chữ cái)*/
+--------------------------------------------------------------------------------------
+a-
+select p.PROF_ID, p.PROF_FNAME, s.STUD_ID, s.STUD_FNAME, c.CLASS_ID, c.CLASS_NAME from PROFESSOR as p 
+    left join CLASS as c on p.PROF_ID = c.PROF_ID
+    left join ENROLL as e on c.CLASS_ID = e.CLASS_ID
+    left join STUDENT as s on e.STUD_ID = s.STUD_ID;
+------------------------------------
+b-
+select distinct p.PROF_ID, p.PROF_FNAME, co.COURSE_ID, co.COURSE_NAME from PROFESSOR as p
+    left join CLASS as c on p.PROF_ID = c.PROF_ID
+    left join COURSE as co on c.COURSE_ID = co.COURSE_ID;
+------------------------------------
+c-
+select distinct s.STUD_ID, p.STUD_FNAME, co.COURSE_ID, co.COURSE_NAME from STUDENT as s
+    left join ENROLL as e on s.STUD_ID = e.STUD_ID
+    left join CLASS as c on e.CLASS_ID = c.CLASS_ID
+    left join COURSE as co on c.COURSE_ID = co.COURSE_ID;
+------------------------------------
+d-/*ENROLL as e*/
+CASE
+    WHEN e.GRADE = 'A' THEN 10
+    WHEN e.GRADE = 'B' THEN 8
+    WHEN e.GRADE = 'C' THEN 6
+    WHEN e.GRADE = 'D' THEN 4
+    WHEN e.GRADE = 'E' THEN 2
+    ELSE 0
+END
+------------------------------------
+e-
+select new.STUD_ID, new.STUD_FNAME, (
+    CASE
+        WHEN average(new.number_grade) >= 8 THEN "Good"
+        WHEN average(new.number_grade) >= 5 AND average(new.number_grade) <8 THEN "Average"
+        ELSE "Weak"
+    END as average_rank
+) from (
+    select s.STUD_ID, s.STUD_FNAME
+    CASE
+        WHEN e.GRADE = 'A' THEN 10
+        WHEN e.GRADE = 'B' THEN 8
+        WHEN e.GRADE = 'C' THEN 6
+        WHEN e.GRADE = 'D' THEN 4
+        WHEN e.GRADE = 'E' THEN 2
+        ELSE 0
+    END as number_grade
+    FROM STUDENT as s left join ENROLL as e on s.STUD_ID = e.STUD_ID
+    where s.STUD_ID = xxx
+) as new group by new.STUD_ID;
+------------------------------------
+f-
+select new.CLASS_ID, new.CLASS_NAME, (
+    CASE
+        WHEN average(new.number_grade) >= 8 THEN "Good"
+        WHEN average(new.number_grade) >= 5 AND average(new.number_grade) <8 THEN "Average"
+        ELSE "Weak"
+    END as average_rank
+) from (
+    select c.CLASS_ID, c.CLASS_NAME
+    CASE
+        WHEN e.GRADE = 'A' THEN 10
+        WHEN e.GRADE = 'B' THEN 8
+        WHEN e.GRADE = 'C' THEN 6
+        WHEN e.GRADE = 'D' THEN 4
+        WHEN e.GRADE = 'E' THEN 2
+        ELSE 0
+    END as number_grade
+    FROM CLASS as c left join ENROLL as e on c.CLASS_ID = e.CLASS_ID
+) as new group by new.CLASS_ID;
+------------------------------------
+g-
+select new.COURSE_ID, new.COURSE_NAME, (
+    CASE
+        WHEN average(new.number_grade) >= 8 THEN "Good"
+        WHEN average(new.number_grade) >= 5 AND average(new.number_grade) <8 THEN "Average"
+        ELSE "Weak"
+    END as average_rank
+) from (
+    select co.COURSE_ID, co.COURSE_NAME
+    CASE
+        WHEN e.GRADE = 'A' THEN 10
+        WHEN e.GRADE = 'B' THEN 8
+        WHEN e.GRADE = 'C' THEN 6
+        WHEN e.GRADE = 'D' THEN 4
+        WHEN e.GRADE = 'E' THEN 2
+        ELSE 0
+    END as number_grade
+    FROM COURSE as co
+    left join CLASS as c on co.COURSE_ID = c.COURSE__ID
+    left join ENROLL as e on c.CLASS_ID = e.CLASS_ID
+) as new group by new.COURSE_ID;
\ No newline at end of file