Mysql :Need to get unique field of two row on base of a column having two different value

I have a database structure like this.

this is just dummy table.

studentId | marks | grade |  examId
   1         25      A         23
   1         32      A         24
   2         13      B         24
   2         23      A         25
   2         34      A         26

I want to write a query to get the studentId of students who was present in both the exams. as a result, I should get studentId '1' but not '2' as student with studentId 2 was not present in exam-'23'.

I want studentIds which are present in examId 23 and 24, not any relation with count of records.

Right now I have write this query but it is not best pe开发者_C百科rformance wise.

SELECT studentId FROM students WHERE examId = 23 AND studentId in (select studentId FROM students where examId in (24) ) 

I am expecting query with better performance.

Ok , final answer as per conversation with Nicollof is

SELECT studentId FROM students WHERE examId IN(23, 24) GROUP BY studentId HAVING COUNT(studentId) = 2

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select studentId from students 
where studentId in (23,24)
group by studentId 
having count(studentId) >1

The where clause filter the result and then the group by clause applied hence you will get what you wanted.

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If you want to be able to tell which exams the students must have participated in (as opposed to the already suggested solutions, including all exams) you could just add a where clause

SELECT studentid 
FROM students WHERE examid IN(23, 24) 
GROUP BY studentid HAVING COUNT(*) > 1 

where 23 and 24 are the exam ids of the exams you are interested in

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Assuming the rows are unique for the combination of studentID and examID, you can just try this -

select studentID from table
WHERE examID in (23, 24)
group by studentID
having count(*) > 1

Edited answer to accomodate the examID in (23,24) after seeing the edit in question and comments.