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Don Burleson Blog 


 

 

 


 

 

 

 

 

Converting Oracle rows to columns

Oracle Tips by Burleson Consulting

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


For more details, see my new book "Oracle Tuning: The Definitive Reference". 

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.


 

 

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