Get first/last n records per group by

Question

I have two tables : tableA (idA, titleA) and tableB (idB, idA, textB) with a one to many relationship between them. For each row in tableA, I want

Answer 1

Ensure your "B" table has an index on ( idA, idB ) for optimized order by purposes so for each "A" ID, it can quickly have the "B" order descending thus putting the newest to the top PER EACH "A" ID. Using the MySQL variables, every time the "A" ID changes, it resets the rank back to 1 for the next "A" id.

select 
      B.idA,
      B.idB,
      B.textB
      @RankSeq := if( @LastAGroup = B.idA, @RankSeq +1, 1 ) ARankSeq,
      @LastAGroup := B.idA as ignoreIt
   from
      tableB B
         JOIN tableA A
            on B.idA = A.idA,
      (select @RankSeq := 0, @LastAGroup := 0 ) SQLVars 
   having
      ARankSeq <= 5
   order by
      B.idA,
      B.idB DESC