SQL Locking...

I have a fellow programmer who has this code in a CL program:

Update OEP013W a set a.wkclosr = 
        (select mtclon from OETITLP b where a.wkorder = b.mtordn)
        where a.wkclosr = 0

Where OEP013W is a work file in QTEMP;
OETITLP is a production PF.

If someone is in the record (locking it) this statement fails.

If you do the sub-SELECT statement by itself with someone locking the
record, it doesn't fail.

Does a SELECT in an UPDATE statement have different rules?

It was our understanding that a SELECT did not lock, or require a lock,
and we assumed would not be inhibited by someone else locking a record
in that file.

TIA

Dave