Oracle SQL按级别连接

Question

Can anyone explain the behavior of the below query:-

select level,t.*
from 
( select 'one','two'from dual
  union all
  select 'one','two'from dual
) t
connect by level<=2

There are 2 rows in the inner query. I was expecting 4 rows of output, but i get 6 rows of output. Why is it so and how does this work?

Answer 1

The query starts with one of your two rows and adds both rows, then it continues with the second row and adds both rows again.

Change your query like this:

select level,t.*
from 
( select 'one' from dual
  union all
  select 'two' from dual
) t
connect by level<=2;

This makes it easier to see what happens:

1   one
2   one
2   two
1   two
2   one
2   two

Answer 2

Read this http://docs.oracle.com/cd/B19306_01/server.102/b14200/queries003.htm

When level <= 1 , you will get each of the records 1 time.

When level <= 2 , then you will get each level 1 time (for level 1) + the number of records in the table (That means for this condition 2 records having level 1 + 2*2 records having level 2. This is the reason you are getting 6 records.)

Answer 3

CONNECT BY LEVEL gives following number of rows x+x2+x3+x4+...x^n = Sx^n

where n is number of LEVEL and x is number of rows in a table