More efficient way to write this SQL Statement???

I have a static sql statement I'm using in an SQLRPGLE program. It takes
about 20 seconds to execute and I need to make it faster if at all
possible...any suggestions will be greatly appreciated...

Here is the statement...

Exec SQL
Declare CSZEST1 Cursor For
Select Cast(tcust As Numeric(6,0)), tship, stadtp, tname,
tadr1,
tadr2, tadr3, tste, tpost, tphone
From est
Where tcust not in(Select ccust
From rcm
Where cad3 = (case when :cszESTCity <> ''
then :cszESTCity else
cad3 End)
and cste = (case when :cszESTState<>''
then :cszESTState else
cste End)
and czip = (case when :cszESTZip <> ''
then :cszESTZip else czip
End)
and cmid = 'CM')
and tadr3 = (case when :cszESTCity <> ''
then :cszESTCity else tadr3 End)
and tste = (case when :cszESTState <> ''
then :cszESTState else tste End)
and tpost = (case when :cszESTZip <> ''
then :cszESTZip else tpost End)
and tid = 'ST'
Order By tcust, tship, stadtp;

The data is structured as follows. There are 2 files, RCM and EST. RCM
is the customer master and EST is the address master for customers. So
the key from RCM to EST is CCUST = TCUST.

TSHIP and STADTP together with TCUST form a unique record. What happens
is the user can specify a City, State, and/or Zip. If they are not blank
I need to use them as inclusion criteria. What I want are any records in
EST that meet the City/State/Zip criteria where the master RCM record does
not.

I hope this is enough clear info.
Thanks
Bryce Martin
Programmer/Analyst I
570-546-4777
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