SQL conditional Statements - postgresql

Question

I am attempting to get all the records from a table where trans_type='RM' but if there is no trans_type='RM' i want to return all the records where trans_type = 'AD'
Technically im using xtupls MetaSQL on a PostgreSQL server so a solution using either is great I can upload my metaSQL statement need be but I really just need a way to do
-- Group: lotserial
-- Name: detail
-- Notes:
-- Copyright (c) 1999-2014 by OpenMFG LLC, d/b/a xTuple.
-- See www.xtuple.com/EULA for the full text of the software license.
SELECT ls_number,
ls_notes,
formatlotserialnumberbarcode(ls_number) AS lotserial_barcode,
item_number,
item_descrip1,
item_descrip2,
charass_char_id,
charass_value,
poitem_id,
poitem_vend_item_descrip,
char_name,
formatqty(itemloc_qty) as lotqty,
lshist.*
FROM
itemloc,
ls
JOIN item ON (item_id=ls_item_id)
LEFT JOIN charass ON (charass_target_id=ls_id)
LEFT JOIN "char" ON (char_id=charass_char_id),
lshist (<? value("itemid") ?>,<? value("warehouseid") ?>,ls_number,
<? value("pattern") ?>,<? value("transType") ?>,<? value("startDate") ?>,
<? value("endDate") ?>,<? value("trace") ?>,1)
LEFT JOIN pohead ON(pohead_number=(TRIM(SUBSTRING(lshist_ordernumber FROM '-.*-'),'-')))
LEFT JOIN poitem ON(poitem_pohead_id=pohead_id)
<? if exists('ls_id') ?>
WHERE ls_id=<? value("ls_id") ?>
<? endif ?>
<? if exists('ls_number') ?>
WHERE ls_number=<? value("ls_number") ?>
<? endif ?>
AND lshist_warehous_code='PS'
<? if exists(TRIM(SUBSTRING(lshist_ordernumber FROM '.*-'),'-')='PO')?>
AND poitem_linenumber = CAST(TRIM(SUBSTRING(lshist_ordernumber FROM '[^-]*$'),'-') AS INTEGER)
<? endif ?>
AND ls_id = itemloc_ls_id
AND charass_target_type = 'LS'
/*
<? if exists(lshist_transtype='RM')?>
AND lshist_transtype='RM'
<? elseif exists(lshist_transtype='AD')?>
AND lshist_transtype='AD'
<? elseif exists(lshist_transtype='RL')?>
AND lshist_transtype='RL'
<? elseif exists(lshist_transtype='SH')?>
AND lshist_transtype='SH'
<? elseif exists(lshist_transtype='IM')?>
AND lshist_transtype='IM'
<? elseif exists(lshist_transtype='TR')?>
AND lshist_transtype='TR'
<? elseif exists(lshist_transtype='RP')?>
AND lshist_transtype='RP'
<? endif ?>

Answer 1

You could use a common table expression:
WITH rm AS (
SELECT * FROM my_table WHERE trans_type = 'RM'
)
SELECT *
FROM data
UNION ALL
SELECT * FROM my_table
WHERE trans_type = 'AD'
AND NOT EXISTS (
SELECT * FROM rm
)
This will avoid the second scan as can be seen in an EXPLAIN ANALYZE call, but there's still a little overhead compared to making the decision in the client, probably due to the CTE materialization (which is PostgreSQL-specific).
I've benchmarked this for an small data set. There seems to be a 5% - 10% overhead in PostgreSQL over running two queries from pgplsql. So, in most cases, and for simple queries like this one, Laurenz's solution is preferrable.
There may be more complex query setups, where the single query is preferrable to two separate queries, as the single query can re-use intermediate results.

Answer 2

I would just run two queries, the first with WHERE trans_type='RM', and only run a second query with WHERE trans_type='AD' if the first one returns an empty result.
I think that trying to squish that into a single query would make things overly complicated and probably also would not be faster – I cannot think of a way that avoids a second scan.