Question

We have two table where where table A and table B need to be join. Relationship between table A and B is one to many. We can join now using this LEFT JOIN tableB on tableA.aID=tableB.aID. We would like to ensure only one record is join on table B and that is the latest record only. We now get more then one record.

SELECT student.*

FROM   student
       LEFT JOIN (SELECT B1.*
                  FROM medicalRecord AS B1
                         LEFT JOIN medicalRecord AS B2
                                ON B1.studentID = B2.studentID
                                   AND B1.checkUpDate< B2.checkUpDate
                  WHERE  B2.studentID IS NULL) AS b
              ON ( student.studentID = medicalRecord.studentID)

Where student.studentID=45

Answer 1

Let us assume you want to get only the record from the child table with the highest value in the val column. You could use:

SELECT a.*,
       b.*
FROM   a
       LEFT JOIN (SELECT B1.*
                  FROM   b AS B1
                         LEFT JOIN b AS B2
                                ON B1.aID = B2.aID
                                   AND B1.val < B2.val
                  WHERE  B2.aID IS NULL) AS b
              ON ( a.aID = b.aID )  

The idea is to use a subquery to filter the table for the records with the greatest values in the val column, then to join on that.