Case statement on a join in where clause

Question

I am trying to use a CASE statement inside a WHERE clause for joining 2 tables. I want to evaluate POSITION_NBR2 , if it is blank (null) then I want to join A.POSITION_NBR2 = B.POSITION_NBR Else I want to join A.POSITION_NBR = B.POSITION_NBR , however I am getting syntax errors while trying to run the following:

UPDATE #WORKTABLE
SET SLT_MEMBER = B.NAME
FROM PS_GHS_FTE_SLT A,
     PS_EMPLOYEES B,
     #WORKTABLE C
WHERE 
  CASE WHEN A.POSITION_NBR2  <> '' THEN A.POSITION_NBR2 = B.POSITION_NBR
       ELSE A.POSITION_NBR = B.POSITION_NBR
       END
  AND A.DEPTID COLLATE Latin1_General_BIN = C.DEPTID COLLATE Latin1_General_BIN
  AND B.EMPL_STATUS = 'A'

Answer 1

Use proper JOIN syntax and reasonable table aliases:

UPDATE WT
    SET SLT_MEMBER = E.NAME
FROM PS_GHS_FTE_SLT GFS JOIN
     PS_EMPLOYEES E
     ON E.POSITION_NBR = (CASE WHEN GFS.POSITION_NBR2 <> '' THEN GFS.POSITION_NBR2 ELSE GFS.POSITION_NBR
                          END) JOIN
     #WORKTABLE WT
     ON GFS.DEPTID COLLATE Latin1_General_BIN = WT.DEPTID COLLATE Latin1_General_BIN 
WHERE E.EMPL_STATUS = 'A';

I'm generally not a fan of using CASE for ON and WHERE clauses. In this case, though, it is only selectively choosing one column, so it doesn't bother me so much.