Query runs fine in MySQL, but returns empty list in Python

  • #1
Wrichik Basu
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I have a database that looks like this:

1709852202892.png


I want to get the "leaderboard position" of a particular member. The idea is to order the data by the score column, and then retrieve the row position of that particular member id. This is my query in MySQL:
MySQL: 
SET @rowNum := 0;
SELECT rowNum
FROM(
  SELECT (@rowNum := @rowNum + 1) AS rowNum, member_id
  FROM members
  ORDER BY score DESC) tmp
WHERE member_id = 288745911563241;

This works as I want in MySQL, returning the appropriate row number:
1709852458956.png


But, when I try to do the same via mysql.connector in Python 3.12, I get an empty list. This is my code in Python:
Python: 
import mysql.connector

mydb = mysql.connector.connect(host="localhost", user="user_name",
                               password="pwd", database="db_name")
mycursor = mydb.cursor()


def main():
    mycursor.execute("SET @rowNum := 0; "
                     "SELECT rowNum FROM( "
                     "SELECT (@rowNum := @rowNum + 1) AS rowNum, member_id "
                     "FROM members "
                     "ORDER BY score DESC) tmp "
                     "WHERE member_id = 288745911563241;")
    print(mycursor.fetchall())
    mycursor.close()
    mydb.close()


if __name__ == '__main__':
    main()

Any idea why I am facing this?

In case anyone wants to test, this is the code for the dummy database:
SQL: 
CREATE DATABASE db_name;

USE db_name;

CREATE TABLE members (
    member_id BIGINT PRIMARY KEY NOT NULL,
    score BIGINT NOT NULL,
    correct BIGINT NOT NULL,
    wrong BIGINT NOT NULL,
    highest_valid_count BIGINT NOT NULL
);

INSERT INTO members VALUES
    (115366974582, 5, 12, 7, 20),
    (588412336974, 25, 78, 53, 105),
    (144865512587, 2, 98, 96, 50),
    (255418963314, 31, 51, 20, 65),
    (5221479632215, 12, 25, 13, 52),
    (25596418873641, 23, 81, 58, 31),
    (75532184413259, 41, 51, 10, 96),
    (288745911563241, 9, 23, 14, 10);
 
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  • #3
Could be all sorts of reasons why it doesn't work, but a robust (and probably more efficient) way to do this is to use two queries, one to get the member's score and another to COUNT the number of rows with a higher (or higher-or-equal) score.
 
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  • #4
jedishrfu said:
My first thought can execute() function process multiple sql statements?
I tried splitting into two execute() statements, but it was not working. Now I tried it again and it is working. Programming can be spooky at times!
pbuk said:
a robust (and probably more efficient) way to do this is to use two queries, one to get the member's score and another to COUNT the number of rows with a higher (or higher-or-equal) score.
Indeed, that's the best approach. Selecting less data means lesser memory overhead, especially for a bot that's supposed to be online all the time. Thanks a lot!
 
  • #5
I'm not a python expert but don't you need an iterator to process the result as in:

Code: 
iterator = cursor.execute(operation, params=None, multi=True)


See:

Code: 
operation = 'SELECT 1; INSERT INTO t1 VALUES (); SELECT 2'
for result in cursor.execute(operation, multi=True): 
if result.with_rows:   
    print("Rows produced by statement '{}':". format(result.statement))   
    print(result.fetchall()) 
else:   
    print("Number of rows affected by statement '{}': {}".format(result.statement, result.rowcount))

https://dev.mysql.com/doc/connector-python/en/connector-python-api-mysqlcursor-execute.html

?

This would probably be a statement handle in ODBC.

Regards.
 
  • #6
Wrichik Basu said:
I have a database that looks like this:

View attachment 341440

I want to get the "leaderboard position" of a particular member. The idea is to order the data by the score column, and then retrieve the row position of that particular member id.

How do you resolve ties? The RANK() or DENSE_RANK() window functions will resolve ties for you (see here), but you will have to rank all the scores before querying results for individual members. If you need to do that multiple times, then it makes sense to do the calculation once in a temporary table and then query that.

Code: 
DROP TEMPORARY TABLE IF EXISTS ranked_by_score;
CREATE TEMPORARY TABLE ranked_by_score AS
SELECT 
member_id,
score,
RANK() OVER w AS 'rank',
DENSE_RANK() OVER w AS 'dense rank',
ROW_NUMBER() OVER w AS 'row_number' 
FROM members 
WINDOW w AS (ORDER BY score DESC);
 
  • #7
My advice here is for you to abstract this into a higher-level representation of your data (using an ORM or even a query builder). My recommendation here is https://www.sqlalchemy.org/

This will be the first of many if you don't migrate your code to a more maintainable and repeatable structure
 
  • #8
As other's have suggested, by default, executor does not handle multiple statements. The mysql docs: https://dev.mysql.com/doc/connector-python/en/connector-python-api-mysqlcursor-execute.html show that there is a multi=True parameter that you can pass into it, but it's also just as easy to just split them into two execute statements:


Python: 
import mysql.connector

mydb = mysql.connector.connect(host="localhost", user="user_name",
                               password="pwd", database="db_name")
mycursor = mydb.cursor()

def main():
    mycursor.execute("SET @rowNum := 0;")
   
    mycursor.execute("SELECT rowNum FROM( "
                     "SELECT (@rowNum := @rowNum + 1) AS rowNum, member_id "
                     "FROM members "
                     "ORDER BY score DESC) tmp "
                     "WHERE member_id = 288745911563241;")
   
    print(mycursor.fetchall())
   
    mycursor.close()
    mydb.close()

if __name__ == '__main__':
    main()

Honestly, from a code readability perspective, I like this more, anyway.
 

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