How to display an average in a temp table using data from two tables in postgres - sql

Question

So I have two tables. one table is called Students and the other table is called Grades. I'm trying to figure out how to join both tables together using the students ID from STUDENTS and students id in GRADE to get their actually grade. With that information I want to be able to find the average of all those grades for that student to display their average in a temporary table.
This is my code so far using JavaScript.. Thanks if anybody can help.
const { Client } = require('pg');
const client = new Client({
host: 'localhost',
user: 'postgres',
database: 'xxx',
password : 'xxx',
port: xxx,
});
client.connect();
const createStudents = `
CREATE TABLE students (
name varchar,
id uuid NOT NULL,
email varchar,
age int
);
`;
client.query(createStudents, (err, res) => {
if(err){
console.error(err);
return;
}
console.log("Students have been successfully connected");
})
const createGrades = `
CREATE TABLE grades (
name varchar,
id uuid NOT NULL,
class varchar,
email varchar,
grade int
);
`;
client.query(createGrades, (err,res) => {
if(err){
console.error(err);
return;
}
console.log("Grades have been successfully connected");
client.end();
})

Answer 1

This is in need of a Foreign Key between grades and students. A simple design that I think achieves what you want:
CREATE TABLE students (
id integer GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
name varchar NOT NULL,
email varchar,
age int
);
CREATE TABLE grades (
id integer GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
student_id integer NOT NULL REFERENCES students ON UPDATE CASCADE,
class varchar NOT NULL,
grade int NOT NULL
);
Then the query:
SELECT
name, avg(grade) AS avg_grade
FROM
grades
JOIN
students
ON
grades.student_id = students.id
GROUP BY
students.id
The FOREIGN KEY eliminates the duplication of information in your original design e.g name and email in the grades table. It all ensures that a grade can only be entered for a student that exists in the students table. The GENERATED ALWAYS AS IDENTITY sets up an automatically incrementing field that can be used as the Primary Key instead of your design which had no declared PK. REFERENCES students will by default use students.id as the referenced column for the values entered in grades.student_id.