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

Question

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 \"rati

Answer 1

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
INNER JOIN 
(
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  = agg.cat
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.

Answer 2

Use joins right after UPDATE: Reference Manual – 13.2.11 UPDATE Syntax

so UPDATE table1 inner join table2 on .... set table1.foo=value where table2.bla = someothervalue

With these kind of things, always look at the manual. MySql has a proper reference manual, so it shouldn't be that hard to get the right syntax ;)

Answer 3

This is how it is done in mssql, I think mysql is the same or similar:

create table T (id int, ratio float, appearances int)
insert T values (1, null, 2)
insert T values (1, null, 3)

update T
set ratio = cast(appearances as float)/ agg.appearancesSum
from T join (
    select id, sum(appearances) as appearancesSum
    from T
    group by id
) as agg on t.id = agg.id