Question

I have a query from my previous question (link) about a complex select,
and my question is, how can I write an update query for result of this query, to change fet_id to some new value.

Note that this query will select rows from a queue until rcpts_count reaches 1000, (for traffic controlling). more information about this query is, in this question

   SELECT NULL AS msg_id, NULL AS total, NULL AS found
    FROM dual
     WHERE (
      @total :=0
       OR 
      @found :=0
    )
    UNION
      SELECT msg_id, @total AS total, @found :=1 AS found
        FROM messages_queue
      WHERE (
        @total := @total + rcpts_count
      )
    AND @total <1000
    UNION
      SELECT msg_id, rcpts_count AS total, 0 AS found
        FROM messages_queue
      WHERE IF( @found =0, @found :=1, 0 )

Answer 1

If you’re trying to update all the records within the set, you could write a query like this:

UPDATE message_queue mq
INNER JOIN (
   SELECT NULL AS msg_id, NULL AS total, NULL AS found
    FROM dual
     WHERE (
      @total :=0
       OR 
      @found :=0
    )
    UNION
      SELECT msg_id, @total AS total, @found :=1 AS found
        FROM messages_queue
      WHERE (
        @total := @total + rcpts_count
      )
      AND @total <1000
    UNION
      SELECT msg_id, rcpts_count AS total, 0 AS found
      FROM messages_queue
      WHERE IF( @found =0, @found :=1, 0 )
) msgs ON msgs.msg_id = mq.msg_id
SET mq.fet_id = 12345;

However, depending on your activity, this isn’t a safe approach, as the records returned from the query could change between the two requests.

I would recommend one of the following approaches:

• Handle the updates on the application side
• Create a temporary table with the result of the query, and update the table with a join to the temporary table
• Assuming ‘fet_id’ is a key unique to this batch, run the update statement first, then do a simple select based on fet_id.