SQL: Select rows with a column value that occurs at least two times?

Question

Here is the database I'm using: https://ufile.io/72wph

The relational schema for the Academics database is as follows:

DEPARTMENT( deptnum , descrip, instname, deptname, state, postcode)

ACADEMIC( acnum , deptnum*, famname, givename, initials, title)

PAPER( panum , title)

AUTHOR( panum*, acnum* )

FIELD( fieldnum , id, title)

INTEREST( fieldnum*, acnum* , descrip)

Some notes on the Academics database:

● An academic department belongs to one institution ( instname ) and often has many academics. An academic only works for one department.

● Research papers ( PAPER ) are often authored by several academics, and of course an academic often writes several papers ( AUTHOR ).

● A research field ( FIELD ) often attracts many academics and an academic can have interest ( INTEREST ) in several research fields.

With this information I have to list the PANUM of papers having at least two authors.

I'm not sure whether I should be joining the AUTHOR table with the ACADEMIC table, or is there a way to do it with just the AUTHOR table?

Answer 1

Unless I'm missing something, I think you can answer your question with a single query on the AUTHOR table alone:

SELECT panum
FROM AUTHOR
GROUP BY panum
HAVING COUNT(*) >= 2;

Joining to the academic table is not needed, because that table just contains metadata for each author, but has nothing to do with the papers.