I have a query that seems it changes its result randomly. I supose that I'm doing something wrong but when I review my query I don't find it. I hope you could help me.
The query orders the results and I use a variable for getting the row position:
SET @row_number_old :=0; SELECT p.id, p.name, f.id, f.others, s.value, s.old_value, ( @row_number_old := @row_number_old +1 ) AS old_position FROM profile p INNER JOIN other_table f ON f.profile_id = p.id INNER JOIN score s ON s.profile_id = p.id WHERE 1 ORDER BY old_value DESC
Despite the data stored in the DB is always the same, the problem is that 'old_position' value is not always the same in the results. Sometimes it has the correct value (the row position), but another times it has the value of f.id inexplicably.
Is it because of your ORDER BY? It seems that could be the issue. Also, although you may be OK in this query, for future reference you may want to be careful reading and setting in the same SELECT query:
While it is also possible both to set and to read the value of the same variable in a single SQL statement using the := operator, this is not recommended. Section 8.4, “User-Defined Variables”, explains why you should avoid doing this.