Case statement to decode

Question

Hello,
I am used to using case within my select statements, I need to use decode if possible for this one. Its for a record group within in forms, and the case statement is not working. Could someone please help me write this with decode instead of case?
SELECT CASE
       WHEN months_between(sysdate , c.birth_dt) BETWEEN 168 AND 216 THEN 1 --14 to 18
       WHEN months_between(sysdate , c.birth_dt) BETWEEN 217 AND 252 THEN 2 --18 to 21
       WHEN months_between(sysdate , c.birth_dt) BETWEEN 253 AND 780 THEN 3 --21 to 65
       WHEN months_between(sysdate , c.birth_dt) > 780 then 4 --older than 65
       ELSE NULL END
FROM mw_clients cThanks in advance!
Jim P.

Answer 1

Below solution is very easy to write.
But I think that performance will be bad.Not really as dual is internal to the SQL engine so there's little difference between the two...
SQL> ed
Wrote file afiedt.buf
  1  with mw_clients as (select add_months(sysdate,-200) as birth_dt from dual union
  2                      select add_months(sysdate,-220) from dual union
  3                      select add_months(sysdate,-400) from dual)
  4  -- END OF TEST DATA
  5  select 4+
  6         decode(Least(months_between(sysdate , c.birth_dt),168),
  7                168,decode(greatest(months_between(sysdate , c.birth_dt),216),
  8                           216,-3,0)
  9                ,0)+
10         decode(Least(months_between(sysdate , c.birth_dt),217),
11                217,decode(greatest(months_between(sysdate , c.birth_dt),252),
12                           252,-2,0)
13                ,0)+
14         decode(Least(months_between(sysdate , c.birth_dt),253),
15                253,decode(greatest(months_between(sysdate , c.birth_dt),780),
16                           780,-1,0)
17                ,0) as result
18* from mw_clients c
SQL> /
    RESULT
         3
         2
         1
Elapsed: 00:00:00.01
Execution Plan
Plan hash value: 1277652150
| Id  | Operation        | Name | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT |      |     3 |    27 |     9  (34)| 00:00:01 |
|   1 |  VIEW            |      |     3 |    27 |     9  (34)| 00:00:01 |
|   2 |   SORT UNIQUE    |      |     3 |       |     9  (78)| 00:00:01 |
|   3 |    UNION-ALL     |      |       |       |            |          |
|   4 |     FAST DUAL    |      |     1 |       |     2   (0)| 00:00:01 |
|   5 |     FAST DUAL    |      |     1 |       |     2   (0)| 00:00:01 |
|   6 |     FAST DUAL    |      |     1 |       |     2   (0)| 00:00:01 |
Statistics
          1  recursive calls
          0  db block gets
          0  consistent gets
          0  physical reads
          0  redo size
        456  bytes sent via SQL*Net to client
        396  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
          3  rows processed
SQL>
SQL> ed
Wrote file afiedt.buf
  1  with mw_clients as (select add_months(sysdate,-200) as birth_dt from dual union
  2                      select add_months(sysdate,-220) from dual union
  3                      select add_months(sysdate,-400) from dual)
  4  -- END OF TEST DATA
  5  select c.birth_dt,
  6         (select 1 from dual where months_between(sysdate , c.birth_dt) BETWEEN 168 AND 216 union
  7          select 2 from dual where months_between(sysdate , c.birth_dt) BETWEEN 217 AND 252 union
  8          select 3 from dual where months_between(sysdate , c.birth_dt) BETWEEN 253 AND 780 union
  9          select 4 from dual where months_between(sysdate , c.birth_dt) > 780
10         ) as result
11* from mw_clients c
SQL> /
BIRTH_DT                RESULT
06/08/1974 10:15:39          3
06/08/1989 10:15:39          2
06/04/1991 10:15:39          1
Elapsed: 00:00:00.01
Execution Plan
Plan hash value: 706016619
| Id  | Operation        | Name | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT |      |     3 |    27 |     9  (34)| 00:00:01 |
|   1 |  SORT UNIQUE     |      |     4 |       |    12  (84)| 00:00:01 |
|   2 |   UNION-ALL      |      |       |       |            |          |
|*  3 |    FILTER        |      |       |       |            |          |
|   4 |     FAST DUAL    |      |     1 |       |     2   (0)| 00:00:01 |
|*  5 |    FILTER        |      |       |       |            |          |
|   6 |     FAST DUAL    |      |     1 |       |     2   (0)| 00:00:01 |
|*  7 |    FILTER        |      |       |       |            |          |
|   8 |     FAST DUAL    |      |     1 |       |     2   (0)| 00:00:01 |
|*  9 |    FILTER        |      |       |       |            |          |
|  10 |     FAST DUAL    |      |     1 |       |     2   (0)| 00:00:01 |
|  11 |  VIEW            |      |     3 |    27 |     9  (34)| 00:00:01 |
|  12 |   SORT UNIQUE    |      |     3 |       |     9  (78)| 00:00:01 |
|  13 |    UNION-ALL     |      |       |       |            |          |
|  14 |     FAST DUAL    |      |     1 |       |     2   (0)| 00:00:01 |
|  15 |     FAST DUAL    |      |     1 |       |     2   (0)| 00:00:01 |
|  16 |     FAST DUAL    |      |     1 |       |     2   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   3 - filter(MONTHS_BETWEEN(SYSDATE@!,:B1)<=216 AND
              MONTHS_BETWEEN(SYSDATE@!,:B2)>=168)
   5 - filter(MONTHS_BETWEEN(SYSDATE@!,:B1)<=252 AND
              MONTHS_BETWEEN(SYSDATE@!,:B2)>=217)
   7 - filter(MONTHS_BETWEEN(SYSDATE@!,:B1)<=780 AND
              MONTHS_BETWEEN(SYSDATE@!,:B2)>=253)
   9 - filter(MONTHS_BETWEEN(SYSDATE@!,:B1)>780)
Statistics
          1  recursive calls
          0  db block gets
          0  consistent gets
          0  physical reads
          0  redo size
        538  bytes sent via SQL*Net to client
        396  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          4  sorts (memory)
          0  sorts (disk)
          3  rows processed
SQL> ed