MySQL query with specific constraints? - mysql

Question

I'm trying to create MySQL queries to answer some questions after creating the following database:
create table teachers (
teacher_id int primary key auto_increment,
name varchar(50),
surname varchar(50),
dob date,
employment_day date
);
create table students (
student_id int primary key auto_increment,
name varchar(50),
surname varchar(50),
dob date,
no_of_lessons_left int
);
create table aircrafts (
aircraft_id int primary key auto_increment,
manufacturer varchar(50),
serial_number varchar(50), -- or int?
cycles int
);
create table schools (
school_id int primary key auto_increment,
address varchar(50),
opening_date date
);
create table lessons (
lesson_id int primary key auto_increment,
teacher_id int not null,
student_id int not null,
aircraft_id int not null,
school_id int not null,
lesson_date date,
constraint lessons_teacher foreign key(teacher_id) references teachers(teacher_id),
constraint lessons_student foreign key(student_id) references students(student_id),
constraint lessons_aircraft foreign key(aircraft_id) references aircrafts(aircraft_id),
constraint lessons_school foreign key(school_id) references schools(school_id)
);
Below some queries, I tried for the questions using double join:
Questions are :
• How many aircraft are there per each school at some point in time?
I can't figure out how to answer these conditions " per each school at some point of time"
• How many students are attending one particular school?
select count(*) from students s
inner join lessons l on l.student_id = s.student_id
inner join schools sc sc.teacher_id= l.teacher_id
where sc.name = "PilotingSchoolName";
• How many students does each teacher have at some particular moment
select count(*) from students s
inner join lessons l on l.student_id = s.student_id
inner join schools sc sc sc.teacher_id= l.teacher_id;
However, I'm missing the condition " at some particular moment " in the query I wrote.

Answer 1

You have a lessons.lesson_date column, and can limit to one point in time with something like AND lesson_date = '2019-12-08' or a date range with AND lesson_date BETWEEN '2018-01-01' AND '2019-12-08'. If the requirements did not ask you to find a specific exact date, it sounds like they'd just like you to be able to demonstrate how to use lesson_date in WHERE.
In your response to "how many students does each teacher have" problem, you are missing a crucial GROUP BY clause. The query you have now is close, but will instead show the total number of enrolled students. Grouping by teachers.name will complete the query (along with a lesson_date constraint)
select
t.name,
-- count each student only once per teacher
count(DISTINCT s.student_id) AS
from teachers t
-- left join so teachers with no enrolled
-- students will list as zero
left join lessons l on t.teacher_id = l.teacher_id
left join students s ON l.student_id = s.student_id
WHERE
-- A specific date
l.lesson_date = '2019-12-08'
GROUP BY t.name
To get the aircrafts per school at a particular time, the query is very similar, but with different joins.
select
-- List all schools, and count of aircraft per school
s.school_id,
count(DISTINCT l.aircraft_id)
from
schools s
-- left join, so that schools with zero current aircraft
-- are include in the results
left join lessons l ON l.school_id = s.school_id
where
l.lesson_date = '2019-12-08'
-- GROUP BY necessary to get the count per school
GROUP BY s.school_id