Grouping rows from a query

Question

Hi everyone!
I want to know if it's posible to group the rows from a query into a unique row. Example:
SELECT Field_1 FROM Table_1
Returns the following fileds:
Field_1
A
B
C
D
I would want to modify my query so that the result obtained would be a single row containing all the others. In the previous example the result would be "ABCD".
Thanks in advance. Best regards,

Answer 1

-- For anyone who wants to test this,
-- just copy this entire post as is,
-- save it to a .sql file, then start that file.
-- Before doing this,
-- make sure that you don't already have
-- a table named table_1 that would be dropped
-- or a .sql file named query.sql that would be overwritten.
-- If you do, then change the names below to avoid any conflicts.
-- In order to understand what the code is doing,
-- please read all of the comments included in this file,
-- as not everything is displayed when you run it.
-- I have gone into great detail with this
-- because I have posted similar things before
-- and a lot of people have total missed the fact
-- that it is not a static query,
-- that the results are dependent upon the number of rows,
-- and that it works for any number of rows,
-- even when the number of rows is unknonwn.
-- test data:
DROP TABLE table_1
CREATE TABLE table_1
  (field_1 VARCHAR2 (1))
INSERT INTO table_1 (field_1)
VALUES ('A')
INSERT INTO table_1 (field_1)
VALUES ('B')
INSERT INTO table_1 (field_1)
VALUES ('C')
INSERT INTO table_1 (field_1)
VALUES ('D')
COMMIT
SELECT * FROM table_1
-- Running the code below will create and start
-- a file named query.sql which will contain the query below,
-- which will produce the results below that:
-- query that will be created:
-- SELECT MAX(DECODE(ROWNUM,1,field_1,NULL))                                      
-- ||MAX(DECODE(ROWNUM,2,field_1,NULL))                                           
-- ||MAX(DECODE(ROWNUM,3,field_1,NULL))                                           
-- ||MAX(DECODE(ROWNUM,4,field_1,NULL))                                           
-- FROM table_1 GROUP BY NULL;                                                    
-- results that will be produced:
-- ABCD
-- Notice that in this example,
-- there are only four values concatenated
-- because there are only four rows in the table. 
-- If there were more rows, it would concatenate more values. 
-- The number of concatenated values
-- is dependent upon the number of rows.
-- The following dynamic sql uses a single query,
-- which produces and executes a single query,
-- whose number of concatenated values is dependent
-- upon the number of rows in the table.
-- Because the echo is set off in the first line,
-- you won't see this code when you run it,
-- just the query that it creates.
SET     ECHO OFF FEEDBACK OFF HEADING OFF PAGESIZE 0 VERIFY OFF
SPOOL   query.sql
-- This is the start of the single query:
SELECT  text
FROM    (SELECT 1                                             AS orderby,
                'SELECT MAX(DECODE(ROWNUM,1,field_1,NULL))'   AS text
         FROM   DUAL
         UNION
         -- This section is the part that dynamically creates
         -- one additional concatenated value
         -- for each additional row after the first row.
         -- This is just a subquery within the single query.
         SELECT rn                                            AS orderby,
                '||MAX(DECODE(ROWNUM,'||rn||',field_1,NULL))' AS text
         FROM   (SELECT ROWNUM rn
                 FROM   table_1)
         WHERE  rn > 1
         UNION
         SELECT COUNT (*) + 1                                  AS orderby,
                'FROM table_1 GROUP BY NULL;'                  AS text
         FROM   table_1)
ORDER BY orderby;
-- This is the end of the single query.
SPOOL    OFF
START    query
SET      ECHO ON
-- If you want to see the file containing
-- the single query that was created, then just:
-- SQL> EDIT query.sql