How to write a query that joins same table in sql and compares the column values of one table with same column in another table

Write a query that checks for traffic inconsistencies. An inconsistency is when a visit (url, uid, dt, src, rev) involves a source page src that was never visited by user uid. Return the key of the visit (url, uid, dt).

This is the table I am working with:

The primary key is url, uid, dt for the visit table.

Visit:

url	dt	uid	src	rev
A02	05/18/2003	A	A05	20
A03	05/19/2003	B	A01	15
A01	5/20/2004	B	A02	10

I would want it to return me these results:

url	dt	uid	src	rev
A02	05/18/2003	A	A05	20
A01	05/20/2004	B	A02	10

This is because A05 which is the source of A02 was never visited by uid. A03 visit would not be returned because the source was visited by B before.

How would I write a query that returns this?

First, I did try connecting the two Visit tables together in a self Join on the src and url:

Select distinct V1.url, V1.uid
FROM Visit V1, Visit V2
WHERE V1.src = V2.url

But I am unsure where to go from here. I know that group by maybe be needed.

Answer

NOT EXISTS helps find these records.

SELECT *
FROM Visit a
WHERE NOT EXISTS (
  SELECT 1
  FROM Visit b
  WHERE b.url = a.src 
  AND b.uid = a.uid
)

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Answer