Need a single query for all IF conditions

Question

I have the following requirements which looks at two parameter p_batch and p_frequency and frames a select to query from the Employers table.
Can I achieve all these in a single select statement? If so how to do it?
Please help.
Thank you.
IF p_batch = 1 and p_frequency = 2 then
SELECT er.employer_id
FROM employers er
WHERE SUBSTR (er.name, 1, 1) BETWEEN 'A' AND 'M'
AND er.freq_flag = 2
ORDER BY er.name asc;
ELSIF p_batch = 2 and p_frequency = 2 then
Open cursor for processing employer starting with N to Z
SELECT er.employer_id
FROM employers er
WHERE SUBSTR (er.name, 1, 1) BETWEEN 'N' AND 'Z'
AND er.freq_flag = 2
ORDER BY er.name asc;
ELSIF p_batch = 1 and p_frequency = 1 then
Open cursor for processing employer starting with A to M for all ER
SELECT er.employer_id
FROM employers er
WHERE SUBSTR (er.name, 1, 1) BETWEEN 'A' AND 'M'
ORDER BY er.name asc;
ELSIF p_batch = 2 and p_frequency = 1 then
Open cursor for processing employer starting with N to Z for all ER
SELECT er.employer_id
FROM wweb.ptei_employers er
WHERE SUBSTR (er.name, 1, 1) BETWEEN 'N' AND 'Z'
ORDER BY er.name asc;
ELSE
-- Return as no need to send any mails
END IF;

Answer 1

Hi,
Sure, you can do that in a single statement. Here's one way:
SELECT       employer_id
FROM       employers
WHERE       SUBSTR (name, 1, 1) BETWEEN  CASE p_batch
                             WHEN  1  THEN  'A'
                             WHEN  2  THEN  'N'
                           END
                     AND      CASE p_batch
                             WHEN  1  THEN  'M'
                             WHEN  2  THEN  'Z'
                           END
AND       (     (     p_frequency = 2
          AND     freq_flag   = 2
       OR     p_frequency  = 1
ORDER BY  name
;