Ordinal Ranking in MySQL Update - mysql

Question

I've got a table like this:
id name incidence placeRef
1 John 10 1
2 Ann 9 1
3 Paul 9 1
4 Carl 8 1
5 John 4 1
6 Ann 4 1
7 Paul 7 1
8 Carl 1 1
I want to rank these using the ordinal ranking method. Which would add ranks to my table as such:
id name incidence placeRef rank
1 John 10 1 1
2 Ann 9 1 2
3 Paul 9 1 2
4 Carl 8 1 4
5 John 4 1 2
6 Ann 4 1 2
7 Paul 7 1 1
8 Carl 1 1 4
How can this be achieved?
N.B. I am going to answer my own question, but would like to know if anyone has any better solutions as it is a bit hacky; though I found numerous posts recommending hacks for this situation.

Answer 1

You can do this with variables or a correlated subquery. The subquery method looks like this:
select t.*,
(select 1 + count(t2.incidence)
from table t2
where t2.incidence > t.incidence
) as rank
from table t;
The variables method is a bit trickier because you have to remember the number of matching rows with a given value:
select t.*,
(#rn := if(#i = t.incidence, if(#cnt := #cnt + 1, #rn, #rn),
#cnt + if(#i := t.incidence, if(#cnt := 1, #rn, #rn), #rn)
)
) as rank
from table t cross join
(select #i := 0, #rn := 0, #cnt := 1) vars
order by incidence desc;
EDIT:
If you want to update the table, just use update with a join:
update table t join
(<either subquery above>) s
on t.id = s.id
set t.rank = s.rank;

Answer 2

The following works:
UPDATE names
JOIN ( SELECT * FROM names ORDER BY placeRef, incidence DESC ) AS p ON p.id = names.id,
( SELECT #curRank := 0, #nextRank := 0, #prevInc := 9999999999, #prevPlace := 0 ) AS v
SET
names.rank = IF( #prevPlace != p.placeRef, #curRank := 0, 0 ),
names.rank = IF( #prevPlace != p.placeRef, #nextRank := 0, 0 ),
names.rank = IF( #prevInc = p.incidence, #nextRank := #nextRank + 1, #curRank := #nextRank := #nextRank + 1 ),
names.rank = IF( #prevInc = p.incidence, #curRank := #curRank, #curRank := #nextRank ),
names.incidence = #prevInc := names.incidence,
names.placeRef = #prevPlace := names.placeRef;
Explanation:
UPDATE names
1 - Sets the table to be updated
JOIN ( SELECT * FROM names ORDER BY placeRef, incidence DESC ) AS p ON p.id = names.id,
2 - This makes a virtual table with the results ordered, so that rankings can be applied
( SELECT #curRank := 0, #nextRank := 0, #prevInc := 9999999999, #prevPlace := 0 ) AS v
3 - This set some variables that will be used to tell when to incrament and reset the rank
names.rank = IF( #prevPlace != p.placeRef, #curRank := 0, 0 ),
4 - This is a hack that resets the current rank to 0 when MySQL iterates into a new place
names.rank = IF( #prevPlace != p.placeRef, #nextRank := 0, 0 ),
5 - This is a hack that resets the next rank to 0 when MySQL iterates into a new place
names.rank = IF( #prevInc = p.incidence, #nextRank := #nextRank + 1, #curRank := #nextRank := #nextRank + 1 ),
6 - This is a hack that updates the next and current ranks when the current incidence is the same as the previous incidence
names.rank = IF( #prevInc = p.incidence, #curRank := #curRank, #curRank := #nextRank ),
7 - This sets the rank to the same as the last rank when its incidence is the same as the previous or increments the rank if it isn't
names.incidence = #prevInc := names.incidence,
8 - This is a hack that sets a variable to contain the previous incidence, so we can tell what to do in the next itteration
names.placeRef = #prevPlace := names.placeRef;
9 - This is a hack that sets a variable to contain the previous place, so we can tell what to do in the next itteration