从同一个表中选择多个列而不使用表联合

Question

I would like to create a query to select several columns from a table, and show the results as a single column. I know I can do this with this query:

select a from Z
union
select b from Z
union 
select c from Z
union 
select d from Z
....

But in my case, table Z is a sub-query of about 50 lines, that I would not want to copy and paste. So I would like to have this in a query where Z appears only once. I don't know if this is possible.

Do you know a way to do that?

Thank you in advance.

Regards.

Answer 1

From Oracle 11gR1 and up, you could use unpivot operator, ensure, of course that all columns are of the same datatype:

SQL> select val
  2    from (select 1 col1
  3               , 2 col2   
  4               , 3 col3
  5            from dual)
  6  unpivot(
  7    val for col in (col1, col2, col3)
  8  )
  9  ;

       VAL
----------
         1
         2
         3

Answer 2

You can do it using a WITH clause :

with Z as (
    select ... from ... -- <<== Put your big query here
)
select a from Z
union
select b from Z
union 
select c from Z
union 
select d from Z

The with clause at the top makes Z available to the remaining parts of your query without having to repeat yourself.

Here is a demo on sqlfiddle .