Question

I have seen many posts but didn’t get my answer i have a table with structure

 subject_level_id  | tutor_id  | level_id_fk  | subject_id_fk |
  118              | 99        | 4            | 1             |
  119              | 99        | 3            | 2             |
  120              | 99        | 3            | 3             |
  121              |100        | 3            | 1             | 
  122              |100        | 4            | 2             |

I want to extract tutor id’s of tutors which teach 1, 2 or more subjects of a particular level( number of subjects depend upon user selection of subjects there can be but level will be unique for aparticular query)

I have worked out a query with help from other posts but it is very slow and gets worse if number of subjects are more than two because I have 10,000 records of tutors my query is:

select distinct
   a.tutor_id
from
   tutor_preferred_level_subject as a
inner join
   tutor_preferred_level_subject as b
   on a.level_id_fk = b.level_id_fk
where
   a.subject_id_fk = 1 and
   b.subject_id_fk = 10 and
   a.level_id_fk = 3

Suggestion: SELECTING with multiple WHERE conditions on same column

The difference in this question and mine is that I want the tutors who teach exactly both the subjects.

update: yes main concern is performance, this query is working fine for two subjects but for more than two subjects mysql server keeps on processing forever. The index is subject_level_id and there are almost 13000 records in it and the below query returns around 6500 results.

as Jakub Sacha demanded these are the results of

EXPLAIN select distinct
   a.tutor_id
from
   tutor_preferred_level_subject as a
inner join
   tutor_preferred_level_subject as b
   on a.level_id_fk = b.level_id_fk
where
   a.subject_id_fk = 1 and
   b.subject_id_fk = 10 and
   a.level_id_fk = 3



id  select_type     table   type    possible_keys   key     key_len     ref|
1   SIMPLE          a   ALL     NULL            NULL    NULL            NULL 
1   SIMPLE          b   ALL     NULL            NULL    NULL           NULL 

rows    Extra 
12733   Using where; Using temporary
12733   Using where; Distinct

Answer 1

Your query does not seem to be doing what you describe. The join should be on tutor_id and not on level_id_fk.

You need the DISTINCT only if the (level_id_fk, subject_id_fk, tutor_id) combination is not unique:

SELECT                              
    -- DISTINCT
   a.tutor_id
FROM
   tutor_preferred_level_subject AS a
INNER JOIN
   tutor_preferred_level_subject AS b
       ON a.tutor_id = b.tutor_id
WHERE
   a.subject_id_fk = 1 AND
   a.level_id_fk = 3 AND
   b.subject_id_fk = 10 AND
   b.level_id_fk = 3 ;

Additionally, you should add a (unique) index on (level_id_fk, subject_id_fk, tutor_id) for efficiency:

ALTER TABLE tutor_preferred_level_subject
  ADD INDEX level_subject_tutor_IDX
    (level_id_fk, subject_id_fk, tutor_id) ;