What SQL query should I write to extract data from three different columns and at the same time do arithmetics for one of them

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I'm trying to extract some data using SQL queries from different tables I have already created. The tables are as follows:

SQL

CREATE TABLE Students 
(
Student_ID INTEGER PRIMARY KEY,
Enrollment_Year DATE,
Course_Current_Status VARCHAR(18), 
First_Name TEXT,
Last_Name TEXT,
Gender TEXT,
Date_Of_Birth DATE,
Email TEXT,
CourseCode INTEGER REFERENCES Courses(CourseCode)
);

CREATE TABLE Modules 
(
Module_Code INTEGER PRIMARY KEY,
Module_Name TEXT, 
Module_Credits INTEGER,
Module_Level INTEGER,
ConvenerID INTEGER REFERENCES Conveners(ConvenerID)
);

CREATE TABLE Enrollment 
(
Marks_Obtained INTEGER,
Module_Code INTEGER REFERENCES Modules(Module_Code),
Student_ID INTEGER REFERENCES Students(Student_ID),
Program_Year_When_Enrolled TEXT, 
PRIMARY KEY(Module_Code, Student_ID)
);

I want to show three columns with my query:

Student_ID, Average_Second_Year_Marks, Average_Third_Year_Marks, Overall_Marks
What I want to do is extract data for students graduating in 2017 i.e. the Course_Current_Status = 'Graduated-2017'

For second year marks the Enrollment.Program_Year_When_Enrolled = 'Second' And for the third year marks the Enrollment.Program_Year_When_Enrolled = 'Third'. For the overall marks a new column would have to be created by the query i.e. Overall Marks which would be 1/3 of the second year marks and 2/3 of the third year marks.

What I have tried:

What I'm using is as follows:

SQL

SELECT 
Students.Student_ID, 
AVG (Enrollement.Marks_obtained WHERE Enrollment.Program_Year_When_Enorolled = 'Third' ) AS avg_third_year_marks,
AVG (Enrollement.Marks_obtained WHERE Enrollment.Program_Year_When_Enorolled = 'Second' ) AS avg_third_year_marks
SUM (avg_third_year_marks*2/3 +avg_second_year_marks*1/3)
FROM 
Students LEFT JOIN
Enrollment ON Students.Student_ID=Enrollment.Student_ID
WHERE 
Students.Course_Current_Year='Graduated-2017'
GROUP BY 
Students.Student_ID

Posted 7-Apr-19 5:51am

User 14184581

Updated 9-Apr-19 8:25am

Maciej Los

v2

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Steve Grattan · Answer 1 · 2019-04-07T07:01:00

You say you have tried this. I assume this is an attempt at the syntax (rather than it returning incorrect results) as there are errors:

1. Enrollement is not a table name - should be Enrollment
2. Program_Year_When_Enorolled - this is a misspelling isn't it?
3. avg_third_year_marks has been specified as an output column twice
4. Course_Current_Year is not a column in the Students table

Can you rectify and resumbit please. Meanwhile I'll see if I can make it work using some assumptions.

Here's a solution if I understood your problem correctly:

SQL

SELECT *, 
Averages.avg_third_year_marks*2/3 + Averages.avg_second_year_marks*1/3 AS overall 
FROM 
(
	SELECT 
	Student_ID,
	(
		SELECT AVG(Marks_obtained) 
		FROM Students s1 
		LEFT JOIN Enrollment e1 ON s1.Student_ID=e1.Student_ID 
		WHERE s1.Student_ID=s.Student_ID 
		AND Program_Year_When_Enrolled = 'Third'
	) AS avg_third_year_marks,
	(
		SELECT AVG(Marks_obtained) 
		FROM Students s2 
		LEFT JOIN Enrollment e2 ON s2.Student_ID=e2.Student_ID 
		WHERE s2.Student_ID=s.Student_ID 
		AND Program_Year_When_Enrolled = 'Second'
	) AS avg_second_year_marks
	FROM 
	Students s
) AS Averages

"Yes thank you very much this works perfectly. However, the results should not be shown for all students, they should relate only to those students whose Course_Current_Year in the students table is 'Graduated-2017'. Can you please tell me how to place this additional restriction. Thanks again"

I've rearranged the post as I was replying incorrectly and I've now lost your reply. Sorry about that...

As I said in my original post there is no column Students.Course_Current_Year so I'm not sure how you can achieve that. Let me know if you need to correct your table definition first and we'll go from there.