If statement within Where clause

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.