UPDATE Syntax with ORDER BY, LIMIT and Multiple Tables

Question

Learning SQL, sorry if this is rudimentary. Trying to figure out a working UPDATE solution for the following pseudoish-code:

UPDATE tableA 
SET          


        

Answer 1

You can use following query syntax:

update work_to_do as target
   inner join (
      select w. client, work_unit
      from work_to_do as w
         inner join eligible_client as e on e.client = w.client
      where processor = 0
      order by priority desc
      limit 10
   ) as source on source.client = target.client
      and source.work_unit = target.work_unit
   set processor = @process_id;

This works perfectly.

Answer 2

The solution is to nest ORDER BY and LIMIT in a FROM clause as part of a join. This let's you find the exact row to be updated (ta.id) first, then commit the update.

UPDATE tableA AS target
    INNER JOIN (
      SELECT ta.id
      FROM tableA AS ta
        INNER JOIN tableB AS tb ON tb.id = ta.user_id
        WHERE tb.username = '$varName'
        ORDER BY ta.datetime DESC
        LIMIT 1) AS source ON source.id = target.id
    SET col1 = '$var';

Hat tip to Baron Schwartz, a.k.a. Xaprb, for the excellent post on this exact topic: http://www.xaprb.com/blog/2006/08/10/how-to-use-order-by-and-limit-on-multi-table-updates-in-mysql/