Question

NOTICE: this question has a ambiguous concept about ‘degree’. and I have got my answer already.

These days I am struggling with a question like below: you can’t write a function using some programming language; use RAW SQL only.

Given a table articles which contains 2 columns: article_id and tag_name

article_id  |  tag_name
---------------------------
1              C++
1              java
1              python
2              ruby
2              js
3              ruby
4              java
4              python

and an empty table named ‘tag_relations’ having a structure like:

tag1     |   tag2    |  degree
-----------------------------------

NOW, the question comes:

• Write a “RAW SQL” to write values to the table ‘tag_relations’ according to the first table’s content.

For the given data, the output should look like ( I am not sure what the degree is, and the question hasn’t mentioned about that, so, both the answers from Gordon Linoff and Jonathan Leffler are correct to me. ) :

tag1     |   tag2    |  degree
-----------------------------------
java         C++        2
java         python     4
ruby         js         2

---

UPDATED ADDITIONAL info

NOTE 1. Here the ‘degree’ was not described by the question. However I think it’s a measure for the relationships between 2 tags. Since there are values for ‘java’ and ‘C++’:

article_id  |  tag_name
---------------------------
1              C++
1              java

so the degree = 2

and for ‘java’ and ‘python’

article_id  |  tag_name
---------------------------
1              java
1              python
4              java
4              python

so the degree = 4

NOTE 2:
There’s no record for degrees with: ‘python’ and ‘C++’, (I mean this question doesn’t refer to the cases that degree < 2) (Note by editor: this comment is inconsistent with including C++ and Java in the output.)

I have spent a day on this question, and reviewed my knowledge on SQL, but still don’t know how to store variables or loop the records.

Any idea will be appreciated!

Answer 1

What is the degree? It appears to be the maximum article id connecting two things.

This query requires a self join (and an insert):

insert into tag_relations(tag1, tag2, degree)
    select a1.tag_name, a2.tag_name, max(article_id) as maxid
    from articles a1 join
         articles a2
         on a1.article_id = a2.article_id and
            a1.tag_name < a2.tag_name
    group by a1.tag_name, a2.tag_name

The self join finds all pairs of items with the same article_id. The “<” just makes sure that reversing the order does not create a new pair. For any pair, the maximum article_id is taken as the new degree.