Question

i ve a little problem in sql, i can’t reach a way to build the correct statement. i need to include in the result only the row having the maximum “sess” when “etud” and “mod” are equal in the occurrences :

etud|sess|mod|eta
1------1---M1--nv
1------2---M1--v
2------1---M1--nv
3------1---M1--v
4------1---M1--v

now the result i need is :

etud|sess|mod|eta
1------2---M1--v
2------1---M1--nv
3------1---M1--v
4------1---M1--v

Thanks in advance .

Answer 1

This is really just a MAX() aggregate, grouping by etud and mod:

SELECT
  etud,
  MAX(sess) AS sess,
  mod
FROM yourtable
GROUP BY etud, mod

Update after edited quetsion:

Although MySQL will allow you to simply add the column to the query above without adding it to the GROUP BY, the value you get for that column is non-deterministic and if you have multiple possible values, you can’t plan on which one you’ll get back (usually it will be the first one though).

To pull in the remaining eta column matching the MAX(sess) you can join against a subquery.

SELECT
  main.etud,
  maxsess.maxsess AS sess
  main.mod,
  main.eta
FROM
  yourtable
  JOIN (
    SELECT
      etud,
      MAX(sess) AS maxsess,
      mod
    FROM yourtable
    GROUP BY etud, mod
    /* JOIN all 3 columns of the subquery which gets the MAX() against the rest of the table */
  ) maxsess ON main.etud = maxsess.etud AND main.sess = maxsess.maxsess AND main.mod = maxsess.mod