Question

I was doing update of big table in Sybase and had following code (declarations skipped):

-- get minimum id 
set rowcount 1 
select @min_id = id from table1 where id matches_some_condition

set rowcount 10000 -- batch size

select @max_id = id from 
(select id from table1 where id >= @min_id and some_condition) a order by id

-- do updates
while (1 = 1)
 begin
   update table1
   set a = some_value
   where id >= @min_id and id <= @max_id and further_conditions

   waitfor delay '00:00:01'

   select @min_id = @max_id
   select @max_id = id from 
    (select id from table1 where id >= @min_id and further_conditions
    ) a order by id

   if @min_id >= @max_id
      break

 end

So it’s usual idea as you can see: take some ordered column (id) and update rows where id is between N and M. Loop until all rows are updated.

As you might see, in order to get @max_id I used set rowcount 10000 and then select @max_id = id from (select id from table1 where id >= @min_id and further_conditions) a. Now I need to do such update on database which is running Sybase ASE 12.0. The problem is that derived tables are only supported from 12.5, so select id from (select id from table1) a is not working. Any ideas how to fix it, perhaps algorithm can be changed? Of course, I can do needed updates with the help of cursor, but I was wondering if there was some better way.

Answer 1

Try to change code:

 select @max_id = id from 
    (select id from table1 where id >= @min_id and further_conditions
    ) a order by id

to:

 select @max_id = max(id) from 
 from table1 
 where id >= @min_id and further_conditions

or

 select @max_id =  id 
 from table1 
 where id >= @min_id 
 and further_conditions
 order by id