Query Issue with select level from dual

Question

Hi,
I have a question regarding this query. The problem seems that when selecting level from dual while including another table the rows returned seem to increase exponentially.
I can add distinct and get the correct number of rows but am worried that this will cause a possible performance issue. Am I using the level option wrong?
I have included details below.
There are 4 rows in tbl_incidents
When I run the following queries I get rows returned based on the total number of rows
select start_date + level - 1, tbl_incidents.incident_id, level
from dual, tbl_incidents
where incident_id = 6
connect by level <= 1;
returns 1 row
select start_date + level - 1, tbl_incidents.incident_id, level
from dual, tbl_incidents
where incident_id = 6
connect by level <= 2;
returns 5 rows
select start_date + level - 1, tbl_incidents.incident_id, level
from dual, tbl_incidents
connect by level <= 3 and incident_id = 6;
returns 21 rows
select start_date + level - 1, tbl_incidents.incident_id, level
from dual, tbl_incidents
connect by level <= 4 and incident_id = 6;
returns 85 rows
select start_date + level - 1, tbl_incidents.incident_id, level
from dual, tbl_incidents
connect by level <= 5 and incident_id = 6;
returns 341 rows
So with
     r being the number of rows in tbl_incidents and
     l being the number used in the connect by for level and
     q being the number of rows returned by the query
     it appears that
q(l) = r * q(l-1) + 1
level 2:     4 * 1 + 1 = 5
level 3:     4 * 5 + 1 = 21
level 4:     4 * 21 + 1 = 85
level 5:     4 * 85 + 1 = 341
Thanks much,
Nora

Answer 1

Hi,
The dual table is used when you want to do something in SQL when you are not otherwise using a table.
Generating a "counter table" of the integers 1, 2, 3,..., X is an example
SELECT  LEVEL   AS n
FROM    dual
WHERE   LEVEL   <= x;There is never any point in joining dual to another table, as in
select  start_date + level - 1
,       tbl_incidents.incident_id
,       level
from    dual
,       tbl_incidents
where    incident_id = 6
connect by  level <= x;You will always get the same more easily by just eliminating dual:
select  start_date + level - 1
,       incident_id
,       level
from    tbl_incidents
where    incident_id = 6
connect by  level <= x;It is quite useful and common to join a counter-table to a real table, like this cross-join:
WITH    counter_table  AS
    SELECT  LEVEL   AS n
    FROM    dual
    WHERE   LEVEL   <= x
select  start_date + n - 1
,       incident_id
,       n
from    tbl_incidents
,       counter_table
where    incident_id = 6