match value against value in other table

[RFH Reseller: Rappie]

I don't see the valid column in the where clause?

Your right, the active and valid names got mixed up. Should be:

UPDATE Table1 set valid = 'false'
WHERE valid = 'true' AND code NOT IN
(SELECT code FROM Table2)

[Staff: Warren Ashcroft]

Is the valid column a varchar column? Normally those columns should be bit columns.

WHERE valid = 1

or

WHERE valid = true (notice no string quotes)

In any case, an index can't do any harm.

[RFH Customer: webb0]

Adding indexes really helped too. One more question though, should I index the `valid` colum as well? Because of the use in the WHERE clause.

Was that a typo and you really mean because of the use of Valid in the UPDATE clause ? If so, I'd say no - dont index it - it'll slow down your query if anything.

[RFH Customer: webb0]

Whoops - just noticed the last 2 posts on this thread.
Seem to recall from somewhere that indexing field that only holds a couple of values isnt worth indexing, but as Warren says it probably wont do any harm in this case

[RFH Reseller: askjim]

Adding indexes will speed up queries significant but does add over head as when inserting / updating data then data needs updating in both the table and index. The overhead will be neglible in comparison to the improvements in selecting data.

You need to analyse the queries that your will be running to determine where you need to put indexes.