PhilipDeFraties/intermediate_sql.md

## intermediate_sql.md

      
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              intermediate_sql.md
            
          
    Intermediate SQL Workshop


Run psql
You may need to run CREATE DATABASE intermediate_sql;
To exit this shell, you can run CTRL d

Create tables

Let's create some tables in the database.

CREATE TABLE students(id SERIAL, name TEXT);
CREATE TABLE classes(id SERIAL, name TEXT, teacher_id INT);
CREATE TABLE teachers(id SERIAL, name TEXT, room_number INT);
CREATE TABLE enrollments(id SERIAL, student_id INT, class_id INT, grade INT);


Add data

Let's insert some students.
INSERT INTO students (name)
VALUES ('Penelope'),
       ('Peter'),
       ('Pepe'),
       ('Parth'),
       ('Priscilla'),
       ('Pablo'),
       ('Puja'),
       ('Patricia'),
       ('Piper'),
       ('Paula'),
       ('Pamela'),
       ('Paige'),
       ('Peggy'),
       ('Pedro'),
       ('Phoebe'),
       ('Pajak'),
       ('Parker'),
       ('Priyal'),
       ('Paxton'),
       ('Patrick');

Let's add some teachers.
INSERT INTO teachers (name, room_number)
VALUES ('Phillips', 456),
       ('Vandergrift', 120),
       ('Mauch', 101),
       ('Patel', 320),
       ('Marquez', 560),
       ('Boykin', 200),
       ('Phlop', 333),
       ('Pendergrass', 222),
       ('Palomo', 323),
       ('Altshuler', 543),
       ('Aleman', 187),
       ('Ashley', 432),
       ('Bonacci', 399),
       ('Brazukas', 287),
       ('Brockington', 299),
       ('Brizuela', 376),
       ('Burkhart', 199),
       ('Choi', 463),
       ('Shah', 354),
       ('Dimaggio', 251);

Let's add some classes.
INSERT INTO classes (name, teacher_id)
VALUES ('Cooking Pasta', 1),
       ('Yoga', 1),
       ('How to Guitar', 2),
       ('Gym', 3),
       ('Football', 4),
       ('Calculus', 5),
       ('Fruit', 6),
       ('Social Studies', 7),
       ('English', 8),
       ('Programming', 9),
       ('Singing', 10),
       ('Fashion', 11);

Lastly, let's add some enrollments!
INSERT INTO enrollments (student_id, class_id, grade)
VALUES (1, 1, 60),
       (2, 2, 70),
       (2, 4, 100),
       (3, 2, 74),
       (4, 3, 82),
       (5, 3, 45),
       (5, 4, 50),
       (7, 11, 62),
       (7, 10, 76),
       (7, 9, 81),
       (7, 8, 91),
       (8, 8, 84),
       (9, 8, 88),
       (9, 7, 83),
       (10, 7, 93),
       (10, 5, 95),
       (11, 5, 95),
       (11, 11, 80),
       (11, 6, 95),
       (11, 1, 94),
       (11, 2, 60),
       (12, 6, 55),
       (13, 7, 97),
       (14, 10, 86),
       (15, 9, 77),
       (15, 4, 93),
       (15, 1, 73),
       (16, 2, 79),
       (16, 6, 73),
       (17, 7, 86),
       (17, 8, 91),
       (17, 9, 93),
       (18, 10, 94),
       (19, 4, 84),
       (20, 1, 85),
       (20, 11, 89),
       (20, 3, 98);

Practice!!


List all the students and their classes
List all the students and their classes and rename the columns to "student" and "class"

ANSWER:
SELECT students.name AS student, classes.name AS class FROM students 
INNER JOIN enrollments ON students.id = enrollments.student_id 
INNER JOIN classes ON enrollments.class_id = classes.id;


List all the students and their average grade

ANSWER:
SELECT students.name student, avg(enrollments.grade) avg_grade FROM students 
INNER JOIN enrollments ON students.id = enrollments.student_id 
GROUP BY students.name;


List all the students and a count of how many classes they are currently enrolled in

ANSWER:
SELECT students.name student, count(*) number_of_classes FROM students 
INNER JOIN enrollments ON students.id = enrollments.student_id 
GROUP BY students.name;


List all the students and their class count IF they are in more than 2 classes

ANSWER:
SELECT students.name student, count(*) number_of_classes FROM students 
INNER JOIN enrollments ON students.id = enrollments.student_id 
GROUP BY students.name 
HAVING COUNT(*)>2;


List all the teachers for each student

ANSWER:
SELECT students.name AS student, teachers.name AS teacher FROM students 
INNER JOIN enrollments ON students.id = enrollments.student_id 
INNER JOIN classes ON enrollments.class_id = classes.id
INNER JOIN teachers ON classes.teacher_id = teachers.id;


List all the teachers for each student grouped by each student

ANSWER:
SELECT students.name AS student, teachers.name AS teacher FROM students 
INNER JOIN enrollments ON students.id = enrollments.student_id 
INNER JOIN classes ON enrollments.class_id = classes.id
INNER JOIN teachers ON classes.teacher_id = teachers.id
GROUP BY students.name, teachers.name
ORDER BY students.name;


Find the average grade for a each class

ANSWER:
SELECT classes.name AS class, avg(enrollments.grade) avg_grade FROM classes
INNER JOIN enrollments ON classes.id = enrollments.class_id
GROUP BY classes.name;


List students' name and their grade IF their grade is lower than the average.

ANSWER:
SELECT students.name student, avg(enrollments.grade) avg_grade FROM students 
INNER JOIN enrollments ON students.id = enrollments.student_id 
GROUP BY students.name 
HAVING AVG(enrollments.grade)<(SELECT avg(enrollments.grade) FROM enrollments);