How to do I get the name of subjects pre-requisites? - sql

Question

Here's a rough sketch. I have a pre-requisite table and subject table.
I have a rough idea how I can list the subject code. But I am really unsure on how I can get a query that can list out the name and details of the subject and it's pre requisites.
For example, I would like to write a query that will list out the subjects names and its pre requisite names.
So the resultant would come out as (Well I'll do the concatenating texts later):
"Introduction to Computer is a pre-requsite of Operating Systems".
I'm just wondering how do I extract the names of subjects off these two tables?
CREATE TABLE subjects (
subject_code VARCHAR(7) NOT NULL CONSTRAINT subject_pk PRIMARY KEY,
subject_name VARCHAR(50) NOT NULL,
subject_details TEXT NOT NULL
);
CREATE TABLE SubjectPrerequisite
( Primary_Subject_Code VARCHAR(7) NOT NULL,
Prerequisite_Subject_Code VARCHAR(7) NOT NULL,
CONSTRAINT PK_SubjectPrerequisite PRIMARY KEY (Primary_Subject_Code, Prerequisite_Subject_Code),
CONSTRAINT FK_SubjectPrerequisite_Primary_Subject_Code FOREIGN KEY (Primary_Subject_Code) REFERENCES Subject (Subject_Code),
CONSTRAINT FK_SubjectPrerequisite_Prerequisite_Subject_Code FOREIGN KEY (Prerequisite_Subject_Code) REFERENCES Subject (Subject_Code)
)
//EDIT: Here's what I have so far
SELECT subject_name
FROM SubjectPreRequisite t0
INNER JOIN subjects t1
ON t0.subject_code = s1.prerequisite_subject_code

Answer 1

Assuming you want the total list of subject names, do this query:
select subject_name from subjects
Assuming you want the subjects pre requisitites and subject_code has a relation with Primary_Subject_Code, do thus query:
select s.subject_name, r.Prerequisite_Subject_Code
from subjects s
inner join SubjectPrerequisite r on s.subject_code = r.Primary_Subject_Code
And with your concat:
select r.Prerequisite_Subject_Code ' + is a pre-requisite of ' + s.subject_name as 'Pre-Requisites'
from subjects s
inner join SubjectPrerequisite r on s.subject_code = r.Primary_Subject_Code

Answer 2

I assume (perhaps wrongly) you are looking to concatenate the subject names of the prerequisites into a single row. Below is a SQL Server example of how this can be done:
;WITH Prerequisites AS
( SELECT Primary_Subject_Code, Subject_Name
FROM SubjectPrerequisite
INNER JOIN Subjects
ON Subject_Code = Prerequisite_Subject_Code
)
SELECT Subject_Code,
Subject_Name,
Subject_Details,
STUFF( ( SELECT ',' + Subject_Name
FROM Prerequisites
WHERE Primary_Subject_Code = Subject_Code
FOR XML PATH(''), TYPE
).value('.', 'VARCHAR(MAX)'), 1, 1, '') AS PrerequisiteList
FROM Subjects
I have previously given a full explanation of how the XML PATH Method works here. With a further improvement to my answer pointed out here
See also SQL Server - Possible Pivot Solution?

Answer 3

If you just want to extract the pre-prequsite relationships to 1 level of recursion you could do
SELECT
[original].[subject_code] [OriginalCode]
, [prereq].[subject_code] [Pre-RequisiteCode]
FROM
[subject] [orginal]
LEFT JOIN
[SubjectPrerequisite] [spr]
ON [spr].[Primary_Subject_Code] = [original].[subject_code]
JOIN
[subject] [prereq]
ON [prereq].[subject_code] = [spr].[Prerequisite_Subject_Code]
ORDER BY
[OriginalCode]
, [Pre-RequisiteCode]
If you want to show the recursive chain and concatenate the subjects in some way then a CTE like GarethD's answer is the way to go. However, I suggest doing that with SQL would be wrong for an n-tier application.

Answer 4

Think I solved it:
SELECT t.subject_name + 'is a pre-requisite of' + s.subject_name
FROM subjects s
INNER JOIN pre_requisites r ON s.subject_code = r.subject_code
INNER JOIN subjects t ON t.subject_code = r.pre_requisite_code