Question

From reading the MySQL documentation, I can’t explain the difference between these two queries in phpMyAdmin:

SELECT * FROM f_ean GROUP BY ean HAVING type = 'media'

–> gives me 57059 results

SELECT ean, type FROM f_ean GROUP BY ean HAVING type = 'media'

–> gives me 73201 results

How can the result number of a query be different by only showing different columns?

Answer 1

You should be using WHERE, not HAVING if you’re trying to filter records. HAVING is used to apply the filter after grouping and sorting have happened.

Regardless, the issue lies with how MySQL uses GROUP BY. GROUP BY should be used with an aggregate; MySQL extends the functionality for convenience. You’re receiving different results because of the way it sorts and groups the columns.

MySQL extends the use of GROUP BY so that the select list can refer to nonaggregated columns not named in the GROUP BY clause. This means that the preceding query is legal in MySQL. You can use this feature to get better performance by avoiding unnecessary column sorting and grouping. However, this is useful primarily when all values in each nonaggregated column not named in the GROUP BY are the same for each group.

See extensions to GROUP BY.