Converting Oracle rows to columns

Oracle Database Tips by Donald Burleson

Sometimes we need to write SQL that takes separate row results (on separate lines) and roll them together into a single column.

Also see my notes on non-SQL techniques for displaying multiple columns on a single row.

There are native SQL techniques to display multiple columns onto a single row

Oracle 9i xmlagg

In Oracle 9i we can use the xmlagg function to aggregate multiple rows onto one column:

select
   deptno,
   rtrim (xmlagg (xmlelement (e, ename || ',')).extract ('//text()'), ',') enames
from
   emp
group by
   deptno
;

    DEPTNO ENAMES
---------- ----------------------------------------
        10 CLARK,MILLER,KING
        20 SMITH,FORD,ADAMS,SCOTT,JONES
        30 ALLEN,JAMES,TURNER,BLAKE,MARTIN,WARD

Use 11g SQL pivot for single row output

The SQL pivot operator allows you to take multiple rows and display them on a single line.

select *
from
(select fk_department
   from employee)
   pivot
    (count(fk_department)
      for fk_department in ('INT', 'WEL', 'CEN', 'POL'));

'INT'         'WEL'       'CEN'     'POL'
----------    ----------   ---------- -------
        7            6            0          8

Use SQL within group for moving rows onto one line and listagg to display multiple column values in a single column

In Oracle 11g, we have the within group SQL clause to pivot multiple rows onto a single row. We also a have direct SQL mechanism for non first-normal form SQL display. This allows multiple table column values to be displayed in a single column, using the listagg built-in function :

select
   deptno,
   listagg (ename, ',')
WITHIN GROUP
(ORDER BY ename) enames
FROM
   emp
GROUP BY
   deptno
/
    DEPTNO ENAMES
---------- --------------------------------------------------
        10 CLARK,KING,MILLER
        20 ADAMS,FORD,JONES,SCOTT,SMITH
        30 ALLEN,BLAKE,JAMES,MARTIN,TURNER,WARD

Use the SYS_CONNECT_BY_PATH operator

This article by Younes Naguib describes how to display multiple values from a single column in a single output row. In his example, he displays multiple values of the last name column on a single row. Note his use of the sys_connect_by_path and over operators:

select
   deptno,
   substr(SYS_CONNECT_BY_PATH(lname, ','),2) name_list
from
   (
   select
     lname,
     deptno,
     count(*) OVER ( partition by deptno ) cnt,
     ROW_NUMBER () OVER ( partition by deptno order by lname) seq
   from
     igribun.emp
   where
     deptno is not null)
where
   seq=cnt
start with
   seq=1
connect by prior
   seq+1=seq
and prior
   deptno=deptno;

DEPTNO NAME_LIST
1      Komers,Mokrel,Stenko
2      Hung,Tong
3      Hamer
4      Mansur

Use a Cross join

Matt contributed this handy SQL techniques to pivot one row of several columns into a single column with several row, using the Oracle Cross join syntax. Matt notes that the Cross join "has other uses in conjunction with a WHERE clause to create triangular result sets for rolling totals etc (though thanks to analytic functions those things are very nice and easy)".

SELECT
  ite,
  case
    when ite = 'item1' then item1
    when ite = 'item2' then item2
    when ite = 'item3' then item3
  end as val
FROM
(
  SELECT
    pivoter.ite,
    item1,
    item2,
    item3
  FROM
    someTable
    CROSS JOIN
    (
      select 'item1' as ite from dual
      UNION ALL
      select 'item2' as ite from dual
      UNION ALL
      select 'item3' as ite from dual
    )pivoter
)

Use the Oracle analytic Lag-Over Function

Analytic functions have a pronounced performance improvement since they avoid an expensive self-join and only make one full-table scan to get the results. This site shows an example of using the Oracle LAG function to display multiple rows on a single column:

SELECT
   ename,
   hiredate,sal,LAG (sal, 1, 0)
   OVER (ORDER BY hiredate) AS PrevSal
FROM
   emp
WHERE
    job = 'CLERK';

Use the SQL CASE operator to pivot rows onto one line

You can use the CASE statement to create a crosstab to convert the rows to columns. Below, the Oracle CASE function to create a "crosstab" of the results, such as this example from SearchOracle:

select Sales.ItemKey
     , sum(Sales.QtySold)   as Qty
     , sum(
         case when OH.MOHClass = 'Fixed'
              then OH.Amt
              else .00 end ) as MOHFixed
     , sum(
         case when OH.MOHClass = 'Var'
              then OH.Amt
              else .00 end ) as MOHVar
     , sum(
         case when OH.MOHClass = 'Cap'
              then OH.Amt
              else .00 end ) as MOHCap
from Sales
left outer
join OH
    on Sales.ItemKey = OH.ItemKey
group
    by Sales.ItemKey