How to query the data in a join table by two sets of joined records? - postgresql

Question

I've got three tables: users, courses, and grades, the latter of which joins users and courses with some metadata like the user's score for the course. I've created a SQLFiddle, though the site doesn't appear to be working at the moment. The schema looks like this:
CREATE TABLE users(
id INT,
name VARCHAR,
PRIMARY KEY (ID)
);
INSERT INTO users VALUES
(1, 'Beth'),
(2, 'Alice'),
(3, 'Charles'),
(4, 'Dave');
CREATE TABLE courses(
id INT,
title VARCHAR,
PRIMARY KEY (ID)
);
INSERT INTO courses VALUES
(1, 'Biology'),
(2, 'Algebra'),
(3, 'Chemistry'),
(4, 'Data Science');
CREATE TABLE grades(
id INT,
user_id INT,
course_id INT,
score INT,
PRIMARY KEY (ID)
);
INSERT INTO grades VALUES
(1, 2, 2, 89),
(2, 2, 1, 92),
(3, 1, 1, 93),
(4, 1, 3, 88);
I'd like to know how (if possible) to construct a query which specifies some users.id values (1, 2, 3) and courses.id values (1, 2, 3) and returns those users' grades.score values for those courses
| name | Algebra | Biology | Chemistry |
|---------|---------|---------|-----------|
| Alice | 89 | 92 | |
| Beth | | 93 | 88 |
| Charles | | | |
In my application logic, I'll be receiving an array of user_ids and course_ids, so the query needs to select those users and courses dynamically by primary key. (The actual data set contains millions of users and tens of thousands of courses—the examples above are just a sample to work with.)
Ideally, the query would:
use the course titles as dynamic attributes/column headers for the users' score data
sort the row and column headers alphabetically
include empty/NULL cells if the user-course pair has no grades relationship
I suspect I may need some combination of JOINs and Postgresql's crosstab, but I can't quite wrap my head around it.
Update: learning that the terminology for this is "dynamic pivot", I found this SO answer which appears to be trying to solve a related problem in Postgres with crosstab()

Answer 1

I think a simple pivot query should work here, since you only have 4 courses in your data set to pivot.
SELECT t1.name,
MAX(CASE WHEN t3.title = 'Biology' THEN t2.score ELSE NULL END) AS Biology,
MAX(CASE WHEN t3.title = 'Algebra' THEN t2.score ELSE NULL END) AS Algebra,
MAX(CASE WHEN t3.title = 'Chemistry' THEN t2.score ELSE NULL END) AS Chemistry,
MAX(CASE WHEN t3.title = 'Data Science' THEN t2.score ELSE NULL END) AS Data_Science
FROM users t1
LEFT JOIN grades t2
ON t1.id = t2.user_id
LEFT JOIN courses t3
ON t2.course_id = t3.id
GROUP BY t1.name
Follow the link below for a running demo. I used MySQL because, as you have noticed, SQLFiddle seems to be perpetually busted the other databases.
SQLFiddle