Oracle SQL - Help using Case in a Select Statement

Question

CREATE TABLE student_totalexp2 nologging compress AS
SELECT /*+parallel(a,4)*/ DISTINCT a.member_sk, 
       CASE 
         WHEN b.end_date IS NULL THEN 
           SYSDATE - MIN(TO_DATE(b.start_date,'yyyymm'))
         ELSE 
           (MAX(TO_DATE(b.end_date,'yyyymm')) - MIN(TO_DATE(b.start_date,'yyyymm')))  
       END as days_experience
  FROM student_schools a 
  JOIN rdorwart.position_rd b ON a.member_sk = b.member_sk 
 WHERE days_experience < 730 
 GROUP BY a.member_sk;

SELECT COUNT(*) 
  FROM student_experience; 

Any idea why I keep getting this error: Error report:

SQL Error: ORA-00904: "DAYS_EXPERIENCE": invalid identifier 00904. 00000 - "%s: invalid identifier" *Cause:
*Action:

Answer 1

You cannot reference a alias in the WHERE clause. Either use a subquery, or better just the entire CASE...END into your where clause.

Updated query per OP's comments:

create table student_totalexp2 nologging compress as 
SELECT a.member_sk, 
 SUM(CASE WHEN b.end_date IS NULL
    THEN sysdate 
    ELSE to_date(b.end_date,'yyyymm') 
  END - to_date(b.start_date,'yyyymm')) as days_experience
FROM student_schools a INNER JOIN rdorwart.position_rd b 
  ON a.member_sk = b.member_sk 
GROUP BY a.member_sk
HAVING SUM(
  CASE WHEN b.end_date IS NULL
    THEN sysdate 
    ELSE to_date(b.end_date,'yyyymm') 
  END - to_date(b.start_date,'yyyymm')
  ) < 730;
SELECT COUNT(*) FROM student_experience;