If statement within Where clause - oracle

Question

I am working with a query which contains "IF" statements within a "WHERE" clause. But PL\SQL Developer is giving some errors while executing it. Can anyone please help me with the correct query? Here is the query:
SELECT t.first_name,
t.last_name,
t.employid,
t.status
FROM employeetable t
WHERE IF status_flag = STATUS_ACTIVE then t.status = 'A'
IF status_flag = STATUS_INACTIVE then t.status = 'T'
IF source_flag = SOURCE_FUNCTION then t.business_unit = 'production'
IF source_flag = SOURCE_USER then t.business_unit = 'users'
AND t.first_name LIKE firstname
AND t.last_name LIKE lastname
AND t.employid LIKE employeeid;
I receive the error "ORA-00920: invalid relational operator".
Placing brackets around status_flag = STATUS_ACTIVE results in error "ORA-00907: missing right parenthesis"

Answer 1

CASE might help you out:
SELECT t.first_name,
t.last_name,
t.employid,
t.status
FROM employeetable t
WHERE t.status = (CASE WHEN status_flag = STATUS_ACTIVE THEN 'A'
WHEN status_flag = STATUS_INACTIVE THEN 'T'
ELSE null END)
AND t.business_unit = (CASE WHEN source_flag = SOURCE_FUNCTION THEN 'production'
WHEN source_flag = SOURCE_USER THEN 'users'
ELSE null END)
AND t.first_name LIKE firstname
AND t.last_name LIKE lastname
AND t.employid LIKE employeeid;
The CASE statement evaluates multiple conditions to produce a single value. So, in the first usage, I check the value of status_flag, returning 'A', 'T' or null depending on what it's value is, and compare that to t.status. I do the same for the business_unit column with a second CASE statement.

Answer 2

You can't use IF like that. You can do what you want with AND and OR:
SELECT t.first_name,
t.last_name,
t.employid,
t.status
FROM employeetable t
WHERE ((status_flag = STATUS_ACTIVE AND t.status = 'A')
OR (status_flag = STATUS_INACTIVE AND t.status = 'T')
OR (source_flag = SOURCE_FUNCTION AND t.business_unit = 'production')
OR (source_flag = SOURCE_USER AND t.business_unit = 'users'))
AND t.first_name LIKE firstname
AND t.last_name LIKE lastname
AND t.employid LIKE employeeid;