MySQL 5.6 - DENSE_RANK like functionality without Order By

Question

I have a table like this:

+------+-----------+
|caseID|groupVarian|
+------+-----------+
|1     |A,B,C,D,E  |
+------+-----------+
|2     |A,B,N,O,P  |
+----         


        

Answer 1

Basically, you want to enumerate the variants. If you just want a number, then you could use the minimum id:

select t.*, min_codeId as groupVariantId
from t join
     (select groupVariant, min(codeId) as min_codeId
      from t
      group by groupVariant
     ) g
     on t.groupVariant = g.groupVariant;

But that is not quite what you want. You appear to want them enumerated by the order they appear in the data. For that, you need variables. This is a bit tricky, but:

select t.*, rn as groupVariantId
from t join
     (select g.*,
             (@rn := if(@gv = groupvariant, @gv,
                        if(@gv := groupvariant, @gv+1, @gv+1)
                       )
             ) as rn
      from (select groupVariant, min(codeId) as min_codeId
            from t
            group by groupVariant
            order by min(codeId)
           ) g cross join
           (select @gv := '', @rn := 0) params
     ) g
     on t.groupVariant = g.groupVariant;

Using variables is tricky. One important consideration: MySQL does not guarantee the order of evaluation of expressions in a SELECT. That means that a variable should not be assigned in one expression and then used in another -- because they could be evaluated in the wrong order (another answer has this mistake).

In addition, the order by needs to take place in a subquery. MySQL does not guarantee that the variable assignment occurs before the sorting.