MySQL/SQL: Update with correlated subquery from the updated table itself


I have a generic question that I will try to explain using an example.

Say I have a table with the fields: "id", "name", "category", "appearances" and "ratio"

The idea is that I have several items, each related to a single category and "appears" several times. The ratio field should include the percentage of each item's appearances out of the total number of appearances of items in the category.

In pseudo-code what I need is the following:

  • For each category
    find the total sum of appearances for items related to it. For example it can be done with (select sum("appearances") from table group by category)

  • For each item
    set the ratio value as the item's appearances divided by the sum found for the category above

Now I'm trying to achieve this with a single update query, but can't seem to do it. What I thought I should do is:

update Table T    
set T.ratio = T.appearances /   
select sum(S.appearances)    
from Table S    
where =    

But MySQL does not accept the alias T in the update column, and I did not find other ways of achieving this.

Any ideas?

Best Solution

Following the two answers I received (none of which was complete so I wrote my own), what I eventually did is as follows:

UPDATE Table AS target
select category, appearances_sum
from Table T inner join (
    select category as cat, sum(appearances) as appearances_sum
    from Table
    group by cat
) as agg
where T.category  =
group by category
) as source
ON target.category = source.category
SET target.probability = target.appearances / source.appearances_sum 

It works very quickly. I also tried with correlated subquery but it was much slower (orders of magnitude), so I'm sticking with the join.