updating million rows based on resultset from previous query - mysql

Question

I need to update a table consisting of million rows
there are two tables table1 and table2
SELECT ID
FROM (
select ID from table1 where<condition>
) as result1
INNER JOIN table2 ON result1.field=table2.field
GROUPBY table2.field
HAVING <condtion>
and on this #resultset1 ID, I have to update table 1
UPDATE table1
SET x=true
where ID EXISTS IN (#resultset1)
there are millions of rows in both table. how do i do it?
And Can anyone say whats wrong with this, i am trying some alternative over join
UPDATE table1 t1
SET x=true
WHERE <condition> AND EXISTS(
SELECT* FROM (
SELECT *
FROM table2 t2
WHERE t2.field = t1.field
) AS result
WHERE<condition on resultset field>
);

Answer 1

Output the results of the first query to a file.
Convert the output to update statements, one update per id (using sed or whatever)
Split the statements into separate files, maybe a 1000 per file (using split or whatever)
Run each file as an sql script with a pause (eg 10 seconds) between each execution (to allow logs to update etc and spread the load), using a simple runner script the loops over the files

Answer 2

Why can't you simply do like below. Though I don't really see a need of group by and having provided if this is your actual query.
UPDATE table1
SET x=true
where ID IN (
SELECT ID
FROM (
select ID from table1 where<condition>
) as result1
INNER JOIN table2 ON result1.field=table2.field
GROUPBY table2.field
HAVING <condtion>
)
Another good option would be JOIN both result set and do the UPDATE like
UPDATE table1 t1
JOIN
(
SELECT ID
FROM (
select ID from table1 where<condition>
) as result1
INNER JOIN table2 ON result1.field=table2.field
GROUPBY table2.field
HAVING <condtion>
) X ON t1.ID = X.ID
SET t1.x=true
EDIT:
You can as well store the result of fist query in a temporary table (As already suggested by a_horse_with_no_name) and then do the UPDATE using a JOIN with the temporary table. Somethig like below
create temporary table idtemp(ID INT);
insert into idtemp
SELECT ID
FROM (
select ID from table1 where<condition>
) result1
INNER JOIN table2 ON result1.field=table2.field
GROUPBY table2.field
HAVING <condtion>
Finally do the update like
UPDATE table1 t1
JOIN idtemp it ON t1.ID = it.ID
SET t1.x=true