diff --git a/database/database.sql b/database/database.sql
new file mode 100644
index 0000000..93b3146
--- /dev/null
+++ b/database/database.sql
@@ -0,0 +1,75 @@
+create table if not exists professor (
+  prof_id integer primary key,
+  prof_lname varchar(50),
+  prof_fname varchar(50)
+);
+
+create table if not exists student (
+  stud_id integer primary key,
+  stud_lname varchar(50),
+  stud_fname varchar(50),
+  stud_street varchar(255),
+  stud_city varchar(50),
+  stud_zip varchar(10)
+);
+
+create table if not exists enroll (
+  stud_id integer,
+  class_id integer,
+  grade enum ('A', 'B', 'C', 'D', 'F')
+);
+
+create table if not exists class (
+  class_id integer primary key,
+  class_name varchar(255),
+  prof_id integer,
+  course_id integer,
+  room_id integer
+);
+
+create table if not exists room (
+  room_id integer primary key,
+  room_loc varchar(50),
+  room_cap varchar(50),
+  class_id integer
+);
+
+create table if not exists course (
+  course_id integer primary key,
+  course_name varchar(255)
+);
+
+alter table
+  enroll
+add
+  constraint fk_student_enroll foreign key (stud_id) references student(stud_id);
+
+alter table
+  enroll
+add
+  constraint fk_class_enroll foreign key (class_id) references class(class_id);
+
+alter table
+  enroll
+add
+  primary key (stud_id, class_id);
+
+alter table
+  class
+add
+  constraint fk_prof_class foreign key (prof_id) references professor(prof_id);
+
+alter table
+  class
+add
+  constraint fk_course_class foreign key (course_id) references course(course_id);
+
+alter table
+  class
+add
+  constraint fk_room_class foreign key (room_id) references room(room_id);
+
+alter table
+  room
+add
+  constraint fk_class_room foreign key (class_id) references class(class_id)
\ No newline at end of file
diff --git a/database/query.sql b/database/query.sql
new file mode 100644
index 0000000..7077211
--- /dev/null
+++ b/database/query.sql
@@ -0,0 +1,138 @@
+## 1
+## những cặp student-professor có dạy học nhau và số lớp mà họ có liên quan
+select
+  concat(p.prof_fname, ' ', p.prof_lname) prof_name,
+  concat(s.stud_fname, ' ', s.stud_lname) stud_name,
+  count(*) number_of_classes
+from
+  student s
+  join enroll e on s.stud_id = e.stud_id
+  join class c on c.class_id = e.class_id
+  join professor p on p.prof_id = c.prof_id
+group by
+  prof_name,
+  stud_name;
+
+## 2
+## những course (distinct) mà 1 professor cụ thể đang dạy
+select
+  distinct *
+from
+  course
+  join class c on course.course_id = c.course_id
+  join professor p on c.prof_id = p.prof_id
+where
+  p.prof_id = 1;
+
+## 3
+## những course (distinct) mà 1 student cụ thể đang học
+select
+  distinct course.course_id,
+  course_name
+from
+  course
+  join class c on course.course_id = c.course_id
+  join enroll e on c.class_id = e.class_id
+  join student s on e.stud_id = s.stud_id
+where
+  e.stud_id = 5;
+
+## 4
+## điểm số trung bình của 1 học sinh cụ thể, quy ra lại theo chữ cái và học lực
+select
+  case
+    when avg(score) >= 9 then 'A'
+    when avg(score) >= 7 then 'B'
+    when avg(score) >= 5 then 'C'
+    when avg(score) >= 3 then 'D'
+    else 'F'
+  end as avg_score,
+  case
+    when avg(score) >= 8 then 'GOOD'
+    when avg(score) < 5 then 'WEAK'
+    else 'AVERAGE'
+  end as ranked
+from
+  enroll e1
+  join (
+    select
+      stud_id,
+      case
+        when grade = 'A' then 10
+        when grade = 'B' then 8
+        when grade = 'C' then 6
+        when grade = 'D' then 4
+        when grade = 'E' then 2
+        when grade = 'F' then 0
+      end as score
+    from
+      enroll
+  ) e2 on e1.stud_id = e2.stud_id
+where
+  e1.stud_id = 2
+group by
+  e1.stud_id;
+
+## 5
+## điểm số trung bình của các class (quy ra lại theo chữ cái)
+select
+  e1.class_id class_id,
+  case
+    when avg(score) >= 9 then 'A'
+    when avg(score) >= 7 then 'B'
+    when avg(score) >= 5 then 'C'
+    when avg(score) >= 3 then 'D'
+    else 'F'
+  end as avg_score
+from
+  enroll e1
+  join (
+    select
+      class_id,
+      case
+        when grade = 'A' then 10
+        when grade = 'B' then 8
+        when grade = 'C' then 6
+        when grade = 'D' then 4
+        when grade = 'E' then 2
+        when grade = 'F' then 0
+      end as score
+    from
+      enroll
+  ) e2 on e1.class_id = e2.class_id
+group by
+  e1.class_id;
+
+## 6
+## điểm số trung bình của các course (quy ra lại theo chữ cái)
+select
+  cre.course_id,
+  cre.course_name,
+  case
+    when avg(score) >= 9 then 'A'
+    when avg(score) >= 7 then 'B'
+    when avg(score) >= 5 then 'C'
+    when avg(score) >= 3 then 'D'
+    else 'F'
+  end as avg_score
+from
+  enroll e1
+  join (
+    select
+      class_id,
+      case
+        when grade = 'A' then 10
+        when grade = 'B' then 8
+        when grade = 'C' then 6
+        when grade = 'D' then 4
+        when grade = 'E' then 2
+        when grade = 'F' then 0
+      end as score
+    from
+      enroll
+  ) e2 on e1.class_id = e2.class_id
+  join class c on c.class_id = e1.class_id
+  join course cre on cre.course_id = c.course_id
+group by
+  cre.course_id,
+  cre.course_name
\ No newline at end of file
diff --git a/database/seed.sql b/database/seed.sql
new file mode 100644
index 0000000..cc79d8a
--- /dev/null
+++ b/database/seed.sql
@@ -0,0 +1,158 @@
+# My data examples for the assignment
+# Use at your own risk
+INSERT INTO
+  professor (prof_id, prof_lname, prof_fname)
+VALUES
+  (1, 'Smith', 'John'),
+  (2, 'Doe', 'Jane'),
+  (3, 'Johnson', 'Michael'),
+  (4, 'Williams', 'Emily'),
+  (5, 'Jones', 'David'),
+  (6, 'Garcia', 'Sophia');
+
+INSERT INTO
+  student (
+    stud_id,
+    stud_lname,
+    stud_fname,
+    stud_street,
+    stud_city,
+    stud_zip
+  )
+VALUES
+  (
+    1,
+    'Brown',
+    'Alice',
+    '123 Main St',
+    'Springfield',
+    '12345'
+  ),
+  (
+    2,
+    'Davis',
+    'Bob',
+    '456 Elm St',
+    'Shelbyville',
+    '54321'
+  ),
+  (
+    3,
+    'Miller',
+    'Charlie',
+    '789 Oak St',
+    'Capital City',
+    '67890'
+  ),
+  (
+    4,
+    'Rodriguez',
+    'Daniel',
+    '246 Maple St',
+    'Ogdenville',
+    '24680'
+  ),
+  (
+    5,
+    'Wilson',
+    'Emma',
+    '369 Pine St',
+    'North Haverbrook',
+    '36912'
+  ),
+  (
+    6,
+    'Martinez',
+    'Olivia',
+    '159 Cedar St',
+    'Brockway',
+    '15975'
+  );
+
+INSERT INTO
+  class (class_id, class_name, prof_id, course_id)
+VALUES
+  (1, 'Intro to Computer Science', 1, 1),
+  (2, 'Calculus I', 2, 2),
+  (3, 'English Literature', 3, 3),
+  (4, 'Chemistry I', 4, 4);
+
+INSERT INTO
+  room (room_id, room_loc, room_cap)
+VALUES
+  (101, 'Building A', '30'),
+  (102, 'Building B', '40'),
+  (103, 'Building C', '35'),
+  (104, 'Building D', '45');
+
+UPDATE
+  room
+set
+  class_id = 1
+where
+  room_id = 101;
+
+UPDATE
+  room
+set
+  class_id = 2
+where
+  room_id = 102;
+
+UPDATE
+  room
+set
+  class_id = 3
+where
+  room_id = 103;
+
+UPDATE
+  room
+set
+  class_id = 4
+where
+  room_id = 104;
+
+INSERT INTO
+  course (course_id, course_name)
+VALUES
+  (1, 'Computer Science'),
+  (2, 'Mathematics'),
+  (3, 'English'),
+  (4, 'Chemistry');
+
+INSERT INTO
+  enroll (stud_id, class_id, grade)
+VALUES
+  (1, 1, 'A'),
+  (1, 2, 'B'),
+  (2, 1, 'C'),
+  (3, 2, 'A'),
+  (4, 1, 'B'),
+  (4, 2, 'A'),
+  (6, 2, 'C'),
+  (1, 3, 'A'),
+  (1, 4, 'B'),
+  (2, 3, 'C'),
+  (2, 4, 'A'),
+  (5, 1, 'C'),
+  (5, 2, 'A'),
+  (5, 6, 'A');
+
+INSERT INTO
+  room (room_id, room_loc, room_cap)
+VALUES
+  (105, 'Building E', '50'),
+  (106, 'Building F', '55');
+
+INSERT INTO
+  class (
+    class_id,
+    class_name,
+    prof_id,
+    course_id,
+    room_id
+  )
+VALUES
+  (5, 'History of Art', 5, 5, 105),
+  (6, 'Physics I', 6, 6, 106);
\ No newline at end of file
diff --git a/leetcode/1393.Capital Gain Loss.sql b/leetcode/1393.Capital Gain Loss.sql
new file mode 100644
index 0000000..87fbbd1
--- /dev/null
+++ b/leetcode/1393.Capital Gain Loss.sql	
@@ -0,0 +1,7 @@
+select
+  stock_name,
+  sum(if(operation = 'Sell', price, - price)) as capital_gain_loss
+from
+  stocks
+group by
+  stock_name
\ No newline at end of file
diff --git a/leetcode/1907.Count Salary Categories.sql b/leetcode/1907.Count Salary Categories.sql
new file mode 100644
index 0000000..bcb45d1
--- /dev/null
+++ b/leetcode/1907.Count Salary Categories.sql	
@@ -0,0 +1,33 @@
+SELECT
+  "Low Salary" category,
+  SUM(
+    CASE
+      WHEN income < 20000 THEN 1
+      ELSE 0
+    END
+  ) accounts_count
+FROM
+  Accounts
+UNION
+SELECT
+  "Average Salary" category,
+  SUM(
+    CASE
+      WHEN income >= 20000
+      AND income <= 50000 THEN 1
+      ELSE 0
+    END
+  ) accounts_count
+FROM
+  Accounts
+UNION
+SELECT
+  "High Salary" category,
+  SUM(
+    CASE
+      WHEN income > 50000 THEN 1
+      ELSE 0
+    END
+  ) accounts_count
+FROM
+  Accounts
\ No newline at end of file