如何使用子查询改善/简化查询？

Question

I want to know the maximum formation level of people, and a person can have many formation entries.

Table t_formation

idFormation | fkPerson   | fkLevel     | place
------------------------------------------------
    1       |     1      |     2       | Oxford
    2       |     2      |     1       | PlaySchool
    3       |     1      |     3       | Trinity High
    4       |     1      |     1       | My School
    5       |     2      |     3       | My High

Table a_level

idLevel | orderLevel | formation
-------------------------------------
  1     |   1        |  School
  2     |   3        |  University
  3     |   2        |  High school

What I need to get is the following query or desired query result ( the max order level with formation of each person and the place where they studied that max formation )

fkPerson | maxOrderLevel |  formation    |  place
----------------------------------------------------
      1  |      2        |   Univertity  |  Oxford
      2  |      3        |   High school |  My High

To do that, I made a query with 2 subquerys but it was impossible to create a efficient view.

Query SQL without place, this get max formation of each person

select fkPerson, a_level.orderLevel, a_level.formation
from (
  select fkPerson,  max(a_level.orderlevel) as ordermax
  from t_formation left join a_level on t_formation.fkLevel = a_level.idLevel 
  group by fkPerson
) as form left join a_level on form.ordermax = a_level.orderlevel

Answer 1

Part of the problem is that you can have multiple orderLevel values for each fkPerson so you will have to retrieve the max(orderLevel) for each fkPerson and then perform another join to get the place / formation that is associated with each person.

I see a couple ways that you can get the result.

You can use a subquery to get the max(orderLevel) and then join on your tables to return the desired columns:

select t1.fkPerson, 
  at.MaxOrder,
  t1.Place,
  a.Formation
from t_formation t1
inner join a_level a
  on t1.fkLevel = a.idLevel
inner join
(
  select t.fkPerson, max(a.orderLevel) MaxOrder
  from t_formation t
  left join a_level a
    on t.fkLevel  = a.idLevel
  group by t.fkPerson
) at
  on t1.fkPerson = at.fkPerson
  and at.maxorder = a.orderLevel;

See SQL Fiddle with Demo .

The other way that you could get the result would be to use a query in your WHERE clause to filter the rows that do not have the max(orderLevel) :

select t.fkperson,
  a.orderlevel,
  t.place,
  a.formation
from t_formation t
left join a_level a
  on t.fkLevel = a.idLevel
where a.orderlevel = (select max(orderlevel)
                      from a_level a
                      inner join t_formation t1
                        on a.idlevel = t1.fklevel
                      where t.fkperson = t1.fkperson
                      group by t1.fkperson);

See SQL Fiddle with Demo .

As a side note, your sample data does not seem to be able to give the result that you request. The max orderLevel for fkPerson = 2 is 3 not 2 so the formation will be University not High School .

Answer 2

Why not use

SELECT fkPerson, fkLevel,orderLevel, formation, place
FROM t_formation as a 
LEFT OUTER JOIN a_level as b ON a.fkLevel = b.idLevel
GROUP BY fkPerson HAVING orderLevel = max(orderLevel)
;

?