Cassandra MAX function returning mismatched rows - cassandra

Question

Hi I am trying to get the max coauthor publication from a table in Cassandra, however its returning me mismatched rows when I query
select coauthor_name, MAX(num_of_colab) AS max_2020 from coauthor_by_author where pid = '40/2499' and year=2020;.
It returns:
which is wrong because 9 belongs to another coauthor.
Here is my create statement for the table:
CREATE TABLE IF NOT EXISTS coauthor_by_author (
pid text,
year int,
coauthor_name text,
num_of_colab int,
PRIMARY KEY ((pid), year, coauthor_name, num_of_colab)
) WITH CLUSTERING ORDER BY (year desc);
As proof, here is part of the original table:
As you can see Abdul Hanif Bin Zaini number publication as coauthor should only be 1.

Answer 1

The MAX() function is working as advertised but I think your understanding of how it works is incorrect. Let me illustrate with an example.
Here is the schema for my table of authors:
CREATE TABLE authors_by_coauthor (
author text,
coauthor text,
colabs int,
PRIMARY KEY (author, coauthor)
)
Here is a sample data of authors, their corresponding co-authors, and the number of times they collaborated:
author | coauthor | colabs
---------+-----------+--------
edda | ramakanta | 5
edda | ruzica | 9
anita | dakarai | 8
anita | sophus | 12
anita | uche | 4
cassius | ceadda | 14
cassius | flaithri | 13
Anita has three co-authors:
cqlsh> SELECT * FROM authors_by_coauthor WHERE author = 'anita';
author | coauthor | colabs
--------+----------+--------
anita | dakarai | 8
anita | sophus | 12
anita | uche | 4
And the top number of collaborations for Anita is 12:
SELECT MAX(colabs) FROM authors_by_coauthor WHERE author = 'anita';
system.max(colabs)
--------------------
12
Similarly, Cassius has two co-authors:
cqlsh> SELECT * FROM authors_by_coauthor WHERE author = 'cassius';
author | coauthor | colabs
---------+----------+--------
cassius | ceadda | 14
cassius | flaithri | 13
with 14 as the most collaborations:
cqlsh> > SELECT MAX(colabs) FROM authors_by_coauthor WHERE author = 'cassius';
system.max(colabs)
--------------------
14
Your question is incomplete since you haven't provided the full sample data but I suspect you're expecting to get the name of the co-author with the most collaborations. This CQL query will NOT return the result you're after:
SELECT coauthor_name, MAX(num_of_colab)
FROM coauthor_by_author
WHERE ...
In SELECT coauthor_name, MAX(num_of_colab), you are incorrectly assuming that the result of MAX(num_of_colab) corresponds to the coauthor_name. Aggregate functions will only ever return ONE row so the result set only ever contains one co-author. The co-author Abdul ... just happens to be the first row in the result so is listed with the MAX() output.
When using aggregate functions, it only makes sense to specify the function in the SELECT statement on its own:
SELECT function(col_name) FROM table WHERE ...
Specifying other columns in the query selectors is meaningless with aggregate functions. Cheers!