如何查询列中具有特定值的最新记录

Question

I have a DB of schools (there are only 3). In the DB, records are submitted regarding their performance.

I want an option for the user to query only the most recent entries for each school.

At the moment - I can query the most recent overall...

$result = mysqli_query($con,"SELECT * FROM reports, academicyears,schools,terms
WHERE reports.report_id=(SELECT MAX(report_id) FROM reports WHERE school_id=1 OR school_id=2 OR school_id=3)
AND academicyears.ay_id=reports.ay_id
AND schools.school_id=reports.school_id
AND terms.term_id=reports.term_id
ORDER BY reports.datesubmitted DESC

But I would like to be able to find the most recent for School 1 & School 2 & School 3 at the same time.

I can see the OR isn't giving me what I want... it just returns the latest record! What should the query be?

Also - the next step is then find the most recent records without having to name the school ids at all. As the number of schools grow - that would be totally inefficient! Is that possible?

Answer 1

A query like this should do the trick. It's three separate queries, 1 for each school, put together by the UNION command:

FROM (
    (
        SELECT * 
        FROM reports, academicyears, schools, terms
        WHERE reports.report_id=(
            SELECT MAX(report_id) 
            FROM reports 
            WHERE school_id=1
        )
        AND academicyears.ay_id=reports.ay_id
        AND schools.school_id=reports.school_id
        AND terms.term_id=reports.term_id
    ) UNION (
        SELECT * 
        FROM reports, academicyears, schools, terms
        WHERE reports.report_id=(
            SELECT MAX(report_id) 
            FROM reports 
            WHERE school_id=2
        )
        AND academicyears.ay_id=reports.ay_id
        AND schools.school_id=reports.school_id
        AND terms.term_id=reports.term_id
    ) UNION (
        SELECT * 
        FROM reports, academicyears, schools, terms
        WHERE reports.report_id=(
            SELECT MAX(report_id) 
            FROM reports 
            WHERE school_id=3
        )
        AND academicyears.ay_id=reports.ay_id
        AND schools.school_id=reports.school_id
        AND terms.term_id=reports.term_id
    )
) AS Latest
ORDER BY datesubmitted DESC

Technically this is running 6 queries in 1, so it will not be terribly efficient. As the number of entries gets larger, this query will get slower.

Answer 2

Youre looking for window functions which aren't in mysql but can be emulated as follows:

SELECT
  *
FROM
  reports, academicyears,schools,terms   ,
  (
    SELECT 
      GROUP_CONCAT(top_codes_per_group) AS top_codes
    FROM
     (
       SELECT 
      SUBSTRING_INDEX(GROUP_CONCAT(report_id.   ORDER By datesubmitted DESC), ',', 5) AS top_codes_per_group
    FROM
      reports, academicyears,schools,terms       
      WHERE school_id=1 OR school_id=2 OR.   school_id=3)
AND academicyears.ay_id=reports.ay_id
AND schools.school_id=reports.school_id
AND terms.term_id=reports.term_id GROUP BY

  ) s_top_codes_per_group
  ) s_top_codes
WHERE
  FIND_IN_SET(Code, top_codes)
ORDER BY
 School,
 Datesubmitted DESC
;

This gives the top 5 reports per school.

Answer 3

Thanks for all help. One of the solutions offered suggested a UNION query. This resulted in a 'duplicate column' error. So, I renamed the columns within the database and updated the query language and it now works!

$result = mysqli_query($con,"SELECT * FROM (
    (
        SELECT *
        FROM reports, academicyears, schools, terms 
        WHERE reports.report_id=(
            SELECT MAX(report_id) 
            FROM reports 
            WHERE rep_school_id=1
        )
        AND academicyears.ay_id=reports.rep_ay_id
        AND schools.school_id=reports.rep_school_id
        AND terms.term_id=reports.rep_term_id
    ) UNION (
        SELECT *
        FROM reports, academicyears, schools, terms 
        WHERE reports.report_id=(
            SELECT MAX(report_id) 
            FROM reports 
            WHERE rep_school_id=2
        )
        AND academicyears.ay_id=reports.rep_ay_id
        AND schools.school_id=reports.rep_school_id
        AND terms.term_id=reports.rep_term_id
    ) UNION (
        SELECT *
        FROM reports, academicyears, schools, terms 
        WHERE reports.report_id=(
            SELECT MAX(report_id) 
            FROM reports 
            WHERE rep_school_id=3
        )
        AND academicyears.ay_id=reports.rep_ay_id
        AND schools.school_id=reports.rep_school_id
        AND terms.term_id=reports.rep_term_id
    )
) AS Latest
ORDER BY datesubmitted DESC
");?>

Thanks again for anyone who helped.