How to fetch odd columns in oracle - oracle

Question

How to fetch odd columns in Oracle using a query when number of columns and name of columns are not known?
E.g.:
I need to get output in below format
Column1 column3 column5 column7
And so on....

Answer 1

You need to use the dynamic queries in the procedure as follows:
SQL> CREATE OR REPLACE PROCEDURE ODD_COLUMNS (
2 TABLE_NAME_P IN VARCHAR2,
3 DATAA OUT SYS_REFCURSOR
4 ) AS
5 V_SQL VARCHAR2(4000);
6 BEGIN
7 SELECT
8 'SELECT '
9 ||
10 LISTAGG(COLUMN_NAME, ',') WITHIN GROUP(
11 ORDER BY
12 COLUMN_ID
13 )
14 || ' FROM "'
15 || TABLE_NAME_P
16 || '"'
17 INTO V_SQL
18 FROM
19 USER_TAB_COLS
20 WHERE
21 TABLE_NAME = TABLE_NAME_P
22 AND MOD(COLUMN_ID, 2) = 1;
23
24 OPEN DATAA FOR V_SQL;
25
26 END ODD_COLUMNS;
27 /
Procedure created.
SQL>
Now, Let's test it:
SQL> variable rc refcursor;
SQL> exec ODD_COLUMNS('EMP',:rc);
PL/SQL procedure successfully completed.
SQL> print rc;
EMP_ID E
---------- -
10 N
20 Y
SQL>
SQL> exec ODD_COLUMNS('MY_TABLE1',:rc);
PL/SQL procedure successfully completed.
SQL> print rc;
ID REQ_QTY
---------- ----------
1001 10
1001 20
1001 30
1002 40
1003 10
1003 20
6 rows selected.
SQL>
Cheers!!

Answer 2

This cannot be done simply, but it is possible using the Oracle data dictionary and some dynamic SQL.
To find out the odd-numbered columns you need to look at the ALL_TAB_COLUMNS view. Column COLUMN_ID sequences the columns 1,2,3. So this will find all the odd-numbered columns in the SCOTT.EMP table:
select column_name, column_id
from all_tab_columns
where owner = 'SCOTT'
and table_name = 'EMP'
and mod(column_id,2) = 1
order by column_id;
This will return something like:
COLUMN_NAME COLUMN_ID
----------- ---------
EMPNO 1
JOB 3
HIREDATE 5
COMM 7
We can use the LISTAGG function to make that into a comma-separated list:
select listagg(column_name,',') within group (order by column_id) as result
from user_tab_columns
where table_name = 'EMP'
and mod(column_id,2) = 1;
RESULT
------
EMPNO,JOB,HIREDATE,COMM
Now we can add to that SQL to generate the select statement you want:
select 'select ' || listagg(column_name,',') within group (order by column_id) || ' from ' || table_name as sql
from user_tab_columns
where table_name = 'EMP'
and mod(column_id,2) = 1
group by table_name;
SQL
---
select EMPNO,JOB,HIREDATE,COMM from EMP
(Note I had to add a group by clause because table_name is not being aggregated by LISTAGG).
You could use that SQL within some PL/SQL code to populate a variable v_sql, then use the DBMS_SQL package to run it. But that is a complex topic in itself and I won't go into it here.