a massive update process to a table

In my example i have two tables as follows

   id                    number
   name              varchar(50)
   aei                  varchar(10)

and another one which has the role of a convserion table (lets call it to_change)

   old_aei          varchar(10)
   new_aei         varchar(10)

i want to build an update statatement which will  replace old_aei with new_aei  in ypotrh
table (aei column)  where aei=old_aei. Please bear in mind that to_change table may contain more than one rows , so the update process should be executed for every line of to_change table.

Thanks very much
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Probably not the most efficient way of performing the update, but worth a try:

update ypotrh y
set y.aei = (select n.new_aei
                  from to_change n
                  where n.old_aei = y.aei)
where exists (select null
                      from to_change n2
                      where n2.old_aei = y.aei)

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If you have may rows to update (100's of thousands or millions) you would probably be better off re-building the table, rather than updating it.
Christoffer SwanströmPartnerCommented:
Instead of updating the table you could do the following:

  ,t2.new_aei AS aei
  ypotrh t1
  to_change t2
  t2.aei = t1.old_aei

Then you can drop the old table and rename the temporary table t to ypotrh.
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basilhs_sAuthor Commented:
fortunately we are talking about a table (to_change) that contains 200 records only. I will try your solution and let you know
Christoffer SwanströmPartnerCommented:
I'm not sure I understand the reason for the where exists clause in Millenniumaire's suggestion. I would simply go for:

update ypotrh y
set y.aei = (select n.new_aei
                  from to_change n
                  where n.old_aei = y.aei)

And yes, for small amounts of data this will quite suffice. My solution above would be better for very large amounts of data.
The where exists clause is required to restrict which rows in ypotrh are updated.

There are likely to be many rows in ypotrh with aei values that don't exist in the to_change table.  Without the where exists clause, the statement will try to update EVERY ROW in ypotrh and for those rows that don't have an old_aei value in to_change, the sub-query will not retrieve a row and this will cause a problem.
I agree with tosse (as I pointed out in 37786012 above) that rather than updating a table with many, many rows it would be better to build a new table.  However, this may be more than a simple create table statement, as it may be necessary to re-create indexes, constraints and triggers on the new table.  Fortunately, in this case you don't need to do this due to the low volumes of data.
basilhs_sAuthor Commented:
perfect and simple solution. thanks
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