Question

Which query will execute faster and which is perfect query ?

SELECT
    COUNT(*) AS count
FROM
    students
WHERE
    status = 1
AND
    classes_id IN(
                    SELECT
                        id
                    FROM
                        classes
                    WHERE
                        departments_id = 1
                );

Or

SELECT
    COUNT(*) AS count
FROM
    students s
LEFT JOIN
    classes c
ON
    c.id = s.classes_id
WHERE
    status = 1
AND
    c.departments_id = 1

I have placed two queries both will output same result. Now I want to know which method will execute faster and which method is correct way ?

Answer 1

You should always use EXPLAIN to determine how your query will run.

Unfortunately, MySQL will execute your subquery as a DEPENDENT QUERY, which means that the subquery will be ran for each row in the outer query. You’d think MySQL would be smart enough to detect that the subquery isn’t a correlated subquery and would run it just once, alas, it’s not yet that smart.

So, MySQL will scan through all of the rows in students, running the subquery for each row, and not utilizing any indexes on the outer query whatsoever.

Writing the query as a JOIN would allow MySQL to utilize indexes, and the following query would be the optimum way to write it:

SELECT COUNT(*) AS count
FROMstudents s
JOIN classes c
  ON c.id = s.classes_id
  AND c.departments_id = 1
WHERE s.status = 1

This would utilize the following indexes:

students(`status`)
classes(`id`, `departements_id`) : multi-column index