mysql逻辑更新一些记录

Question

I have a specific recordset of 48 records (1 day over 1/2 hour slots).

I want to reduce the value of l of the top 24 records (top by l field DESC ) by a pre-known amount and all i have is the date and the amount I want to reduce by (a php var called $int).

Currently, I am reducing all 48 records as such:

UPDATE r SET l = l - $int WHERE thedate = '$kDate'

(updates 48 records)

How do I update the top 24?

Thanks in advance.

Answer 1

UPDATE r 
SET l = l - $int 
WHERE thedate = '$kDate'
order by l desc
limit 24

Answer 2

UPDATE r SET l = l - $int WHERE thedate = '$kDate'
ORDER BY l DESC
LIMIT 24

Answer 3

UPDATE r SET l = l - $int WHERE thedate = '$kDate' ORDER BY thedate DESC LIMIT 24